What is the difference between color and colour?
According to the Oxford English Dictionary, this word comes to us from the Latin word color-em, which was inherited by old French. The o segment in Old French was actually somewhere between [o] and [u] and was represented by the digraph ou. When this syllable became accented, it became fronted, represented by eu (this the Modern French couleur). The corresponding English word of old is the ancestor of hue.
The world was adopted directly into Middle English as colur; later colour from the Old French color, culur, colur; later colour; and finally coulour, which was the preferred Anglo-French spelling. Colour was eventually standardized in post Anglo-French English. Noah Webster's 1828 dictionary is credited with standardizing the latinate color in American English.
"Color" is favored in the United states, and "Colour" is favored (I believe) everywhere else.
Here is a Wikipedia article commenting on "or" vs. "our".