Originally they were a jade green color which I liked. Then the jersey system came along and they changed it to the dark green because that was one of shades of green they used and I guess the medium green and light green didn't work at all.Speaking of which I sometimes wonder why China-Qin is a dark green - IRL Qin ordered his empire's "national color" to be black for auspicious indications (black is the color of water in early Chinese epistemology, and Qin was said to have the virtue of water).
Because when speaking of "Chinese Empire" people will immediately thought of "jade"? I do agree that having a major civ colored in black is very odd.
Some original color schemes of civs were much better like the rose colored Poland. The only one I am glad they changed was Australia, who finally got their correct color scheme.
And the Dutch who are now a correct colored orange, not the brownish orange that they started with.