The SG550 was green to start with, so were AUG and CETME L.
Black is a horrible colour, other than that it does not look as dirty after touched by millions of people. I would much prefer a dark grey or a coyote brown. On the other hand, people are so ingrained with black for some unknown reason ( probably due to TV/movie and the fact that in the old days bluing and parkerization are the only way to go)
On the other hand, anodization sometimes does not look natural with other colours - most of the time brown turns gold.
I also got a feeling that in the old days, mixing green paint is probably more time consuming than getting black.