I think that in current everyday usage, people use "is concentrated" to mean simply "is present in disproportionately large numbers relative to its level of occurrence elsewhere." This usage makes perfect sense to me.
Long ago, "concentrate" had a rather narrow definition and was identified as being strictly an "active" (that is, transitive) verb. In Samuel Johnson's Dictionary of the English Language (1756), for example, the only definition provided is
To drive into a narrow compass.
But in Webster's An American Dictionary of the English Language (1845), concentrated already meant something more general:
Brought to a point or centre; brought to a closer union; reduced to a narrow compass; collected into a closer body.
The meaning of concentrate in Merriam-Webster's Eleventh Collegiate Dictionary (2003) that applies most closely to your examples is the second intransitive-verb definition given there:
Just as raindrops may collect (and become concentrated) in a puddle relative to their numbers on the slightly sloped sidewalk nearby, so may blue people collect (or become concentrated) in low-density rural areas relative to their numbers in other parts of the country, and green people collect (or become concentrated) in cities relative to their numbers elsewhere. It follows that concentrated works in all three examples you give in your question.