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My mollies are about 4 wks old, and I have been able to tell from very early on with this batch. they are orange and black, some mixed some solide each color. With my first batch it was harder to tell and took longer. they were a dalmation molly, and it took awhile for some to start getting darker. So I think it really also depends on the color of your mollies! (mom and dad)
as for "where" I find that the tails are hardest to "see" but generally the color develops everywhere at the same rate.... but considering my two toned fish (half orange/half black) it was obvious they were going to be two different colors, but as they got bigger the black got blacker and the orange got oranger Hope this helps...