I hesitated to post this answer because it is mainly concerned with the origin and historical evolution of the terms "person of color," "persons of color," and "people of color" in the United States. In fact, my answer doesn't answer any of the four specific questions that RaceYouAnytime poses, although (I believe) it does provide some useful context for thinking about them.
In any event, I hope that you will take this answer for what it is—an exploration of the historical and legal underpinnings of "person of color" in U.S. usage. If you are interested solely in responses that focus on addressing RaceYouAnytime's four posted questions, I encourage you not to read it.
Early use of 'person of color' as a legal term
The earliest occurrences of "person of color" in a Google Books search date to the early nineteenth century and involve U.S. laws regulating the slave trade. The earliest of these is "An Act to Prevent the Importation of Certain Persons into Certain States, Where, by the Laws Thereof, Their Admission Is Prohibited" (February 28, 1803), reproduced in John Brice, A Selection of All the Laws of the United States, Now in Force, Relative to Commercial Subjects (1814):
SEC. 1. Be it enacted by the Senate and House of Representatives of the United States of America, in Congress prohibited assembled, That from and after the first day of April next no master or captain of any ship or vessel, or any other person, shall import, or bring, or cause to be imported or brought, any negro, mulatto, or other person of color, not being a native, a citizen, or registered seaman of the United States, or seamen natives of countries beyond the Cape of Good Hope, into any port or place of the United States, which port or place shall be situated in any state which by law has prohibited or shall prohibit the admission or importation of such negro, mulatto, or other person of color, and if any captain or master aforesaid, or any other person, shall import or bring, or cause to be imported or brought into any of the ports or places aforesaid, any of the persons whose admission or importation us prohibited, as aforesaid, he shall forfeit and pay the sum of one thousand dollars for each and every negro, mulatto, or other person of color aforesaid, brought or imported as aforesaid, to be sued for and recovered by action of debt, in any court of the United States; one half thereof to the use of the United States, the other half to any person or persons prosecuting for the penalty; and in any action instituted for the recovery of the penalty aforesaid, the person or persons sued may be held to special bail: Provided always, That nothing contained in this act shall be construed to prohibit the admission of Indians.
The term "person of color" thus began as a catch-all legal term intended to include any person whose ancestry was entirely African (negro), half African (mulatto) or some smaller proportion African, as well as (arguably) any person who had dark skin but came from some part of the world other than Africa—but not including North American Indians.
Within a fairly short period of time, some people who fell within that legal classification began using it as a descriptive category for themselves, as, for example, in this instance from "The Colonization Scheme," in Niles' Weekly Register (November 27, 1819):
Protest and remonstrance of the people of color, of the city and county of Philadelphia, against the plan of colonizing the free people of color of the United States, on the coast of Africa.
At a very numerous meeting of persons of color, on the 16th instant, the following resolutions were unanimously adopted:
Whereas, an address to the citizens of Philadelphia and New York, having been made through the medium of the public papers, by the agents of the American society for colonizing the free people of color on the coast of Africa, which address is made, it is said on behalf of a number of people who are desirous of joining the projected colony in Africa, and who have made application to the American colonization society for permission to be among its first colonists: ...
Resolved, That how clamorous soever a few obscure and dissatisfied strangers among us may be in favor of being made presidents, governors and principals in Africa. there is but one sentiment among the respectable inhabitants of color in this city and county, which is, that it meets their unanimous and decided disapprobation.
Resolved, That the people of color of Philadelphia, now enter and proclaim their solemn protest against the contemplated colony on the shores of Africa, and against every measure that may have a tendency to convey an idea, that they give the project a single particle of countenance or encouragement. [signed,] JAMES FORTEN, chairman.
Subsequently, various federal and state laws in the United States used the same terminology for various race-specific laws and ordinances. Levi Pierce, Miles Taylor & William King, The Consolidation and Revision of the Statutes of the State [of Louisiana], of a General Nature (February 5, 1852), for example, devotes a six-page section of relevant statutory provisions under the heading "Free Persons of Color." The term "person of color" likewise plays a major role in the post–Civil War period, in (for example) the Freedmen's Code (also known as the South Carolina Black Code of 1866).
'Person of color' in U.S. law in 1904
The legal glossary, Judicial and Statutory Definitions of Words and Phrases, Volume 2, Casual—Deposition (1904) devotes a full page to different definitions of "colored person" (and "person of color") in various jurisdictions of the United States, including such thumbnail definitions as these:
As the term "persons of color" has acquired quite as definite a meaning as "negro," "mulatto," etc., and is used in the statute prohibiting vessels sailing for the purpose of engaging in the slave trade, the use of the term in an indictment for the violation of the statute does not render the indictment invalid, as being too indefinite and unintelligible. [U.S. federal court citation omitted.]
The expression "person of color" means negro or mulatto. [U.S. federal court citation omitted.]
A free woman of color" is to be understood to be one of African descent, and synonymous with "free negro." [Mississippi court citation omitted.]
A colored person, or person of color, is a person having one-fourth or more of negro blood. Such a person, by Code 1860, c. 130 § 9, was declared to be a mulatto; but that section only applied to slaves, and not to free negroes. [Virginia court citation omitted.]
"Free person of color" does not mean only a free negro, but may mean persons colored by Indian blood or persons descended from negro ancestors beyond the fourth degree. [North Carolina court citation omitted.]
The term "persons of color" was defined in 13 Stat. 245, to designate all free negroes, mulattoes, and mestizos, all freedmen and freedwomen, and all descendants through either sex of any of these persons. [South Carolina court citation omitted.]
The expression "persons of color" includes all who are descended from negro ancestors of the fourth generation, inclusive, though one ancestor of each generation may have been a white person. [North Carolina court citation omitted.]
The term "person of color," as used in Acts 1838, c. 24, declaring void all marriages between white persons and free negroes and persons of colors, includes only cases where such persons of color are within the third degree. [North Carolina court citation omitted.]
The phrase "persons of color" means "Africans or their descendants, mixed or unmixed. A person who has any perceptible admixture of African blood is generally called a 'colored person.' In affixing the epithet 'colored,' we do not ordinarily stop to estimate the precise shade, whether light or dark, though, where precision is desired, they are sometimes called 'light-colored' or 'dark-colored.' There is no margin between white and colored, and all that are not white are colored." [Ohio court citation omitted.]
In the construction of statutes, the term "colored person," or "person of color," or "colored," as applied to any person, has the same signification as is attached to "negro," which term includes every person having one-eighth or more of negro blood. [Florida court citation omitted.]
In State v. Treadaway (1908), the Louisiana Supreme Court was asked to determine whether a white man and a woman who was one-eighth black had violated a state statute prohibiting "[c]oncubinage between a person of the Caucasian or white race and a person of the negro or black race." The court held that the legislators had not intended for the statute to prohibit relationships between whites and certain "persons of color":
There are no negroes who are not persons of color; but there are persons of color who are not negroes. The term "colored," as applied to race, was given the meaning of the word "negro" for the very purpose of having in the language a term including within its meaning both negroes and descendants of negroes; but the converse is not true. The word "negro" was never adopted into the language for the purpose of designating persons of mixed blood. On the contrary, it was for the purpose, and the sole purpose, of expressing the meaning of persons of the negro race proper; and it can have now a different or more enlarged meaning only by enlarging its original meaning, as was done with the word "colored," and imparting to it a meaning different from that which it originally bore in the language. The legislature might do this, but the statute by which it did it would have authority only in Louisiana, and the word "negro" would still continue to mean the world over, outside of Louisiana, except where its meaning had in a like manner statutorily enlarged, a person of the African race, or possessing the black color and other characteristics of the African.
Clearly there is considerable disagreement in these various sources about how far beyond the core meaning "person of African descent" courts should extend the term "person of color." Opinions ranged from "'person of color' means negro or mulatto" to "'person of color' ...may mean persons colored by Indian blood or people descended from negro ancestors beyond the fourth degree."
'Person of color" in U.S. law in 1945
U.S. federal and state statutes (especially in Southern states) continued to include provisions mentioning "persons [or people] of color." One interesting development that arose from this phenomenon was the extension of the legal definition of "people of color" to refer to people who had no modern African forebears. One relevant discussion of this point appears in U.S. Congress, Senate Committee on Immigration, To Permit All People from India Residing in the U.S. to Be Naturalized: Hearings Before a Subcommittee on S. 236 ... (1945) [combined snippets], in which a spokesperson for a labor union testified against a bill to extend naturalization rights to residents of the United States who had emigrated from India:
The Junior Order United American Mechanics, is opposed to this proposed bill, S. 236. We believe it would be unwise to permit the naturalization of people of India for several reasons, as follows:
The Revised Statutes of the United States prohibit naturalization of people of African descent, or people of color. This is a law, and a well-established one, which has been reviewed and interpreted by the Supreme Court of the United States, and has been upheld. If we remember correctly, this section of the Revised Statutes to which we refer was contested by a Chinaman in several Federal courts, and finally reached the Supreme Court, which denied the Chinaman permission or right to be naturalized. So far as our information goes, no change has been made in this fundamental law, prohibiting people of color to become naturalized citizens of the United States, unless it was done in the repeal of the Chinese exclusion laws. The long precedent of prohibiting the naturalization of people of color, which seems so well established, and a fundamental part of the laws of our Nation, would make it unwise to amend or change this law. There must have been a good reason for prohibiting people of color from becoming naturalized citizens of our country. This law was aimed primarily at people of Africa, but it applies with equal force to people of India, because they have been properly classified as persons of color.
In short, race-conscious white judges and politicians in the United States were reframing a phrase that had originated as a category name for people who had any modern African forebears in order to encompass other people of non-European heritage so that they could be denied the rights and opportunities that U.S. residents of exclusively European heritage enjoyed.
This more expansive notion of "person [or people] of color" may not have had any direct influence on the emergence of "people of color" as an umbrella term for non-white/-Caucasian/-European persons in the 1970s—but there is no mistaking that the sense and coverage of the term in this anti-diversity testimony at a 1945 Congressional hearing are much the same as they are in the pro-diversity rhetoric of the 1970s and later.
'Person of color' in recent decades
The Ngram chart for "person of color (blue line) vs. "persons of color" (red line) vs. "people of color" (green line) vs. "woman of color" (yellow line) vs. "man of color (light blue line) vs. "women of color" (maroon line) vs. "men of color" (light green line) for the period 1950–2008 indicates that, while most of these terms have increased rather modestly in frequency of use since 1950, "people of color" and "women of color" have increased quite substantially—especially since 1980 or so:
The rise of those two phrase, in particular, seems to reflect the popularization of "people of color" and "women of color" as convenient ways to refer to "people other than whites" and "women other than whites," respectively.
Google books finds instances of "people of color" used in a positive or sympathetic modern sense at least as far back as 1975. The earliest is from Edward Murguía, Assimilation, Colonialism, and the Mexican American People (1975) [combined snippets]:
As mentioned before, mode of entry is closely related to the variable of race because since the sixteenth century the people of color have been conquered and colonized by the white race.
Culture is related to race because when the different races originally made contact with one another their cultures were very different. Since the Caucasians had technological and military superiority over the peoples of color, they assumed cultural superiority as well. Although the people of color have learned many of the cultural aspects of Western civilization, they have retained varying amounts of their native culture. For those people of color in the United States who have undergone cultural assimilation to a great extent, cultural differences due to class remain between most of the people of color and most of the Caucasians in America.
And from Marie Foster Branch & Phyllis Perry Paxton, Providing Safe Nursing Care for Ethnic People of Color (1976) [combined snippets]:
For the most part, white majority persons and middle-class-oriented ethnic persons of color will have to supplement their life experiences in a purposeful way in order to become reeducated for ethnic practice. One might term this process “late life education” for practicing nurses. Hopefully, nursing students will receive education about ethnic and cultural diversity in the process of becoming nurses.
It is noteworthy that in Branch & Paxton consistently uses the term "ethnic persons of color"—not "persons of color" alone—perhaps to emphasize the term is not functioning as a way to comprehend multiple degrees of African heritage, but rather as a way to emphasize the non-whiteness of the relevant people of color, regardless of their particular ethnicity.
The term "person of color" has a long and interesting history of use in the United States dating back at least to the early 1800s. In its earliest uses, it seems primarily to have provided lawyers and judges with a way to negotiate the question of what rights and privileges to accord to people who didn't fit into a simple white/black bipartite categorization of U.S. residents—or even into one that occasionally attempted to carve out subcategories for mulatto, quadroon, octoroon, and so on, across multiple generations.
But because "person of color" was inherently a catch-all category for anyone not deemed to be entirely white/European/Caucasian, judges and politicians in various U.S. jurisdictions began to expand it to include persons from a more complex world in which many people had origins other than African or European or some mixture of the two. Differing views of who was and who was not a "person of color" crop up in legal commentaries for more than 150 years, and nothing remotely approaching consensus seems to have emerged from these discussions.
Starting in the 1970s, however, the term "person of color" was adopted as a positive, unifying term for people whose ethnicity was not entirely European or who did not view themselves as white.
That usage dominates present-day awareness of the term—and at least to this point, I have seen no tendency to use "person of color" interchangeably with "colored person." This, I believe, reflects the historical reality that "colored man/woman/person" was widely used in everyday U.S. English for a person of African heritage and was not normally applied to people of Asian, Native American, or Pacific Islander descent. "Person of color," in contrast, was not in common use 50 years ago and would probably have struck most U.S. hearers in 1968 as being quaint, legalistic, or both.
But its out-of-the-mainstream status in the late 1960s probably helped "person of color" catch on with people who wanted a term that wasn't already overloaded with negative connotations in popular usage. Indeed, I imagine that relative few people who use the term today are aware that it emerged in U.S. English two centuries ago as a way of linking people of mixed European and African heritage more closely to their African forebears than to their European ones.