The males of both species turn darker during the spawn. I call it "putting on their tuxedos for the dance". In clear water the males can get jet black. The females remain the same as they are the rest of the year.
A lot of folks think the darker fish they catch are black crappie and the lighter colored fish are white crappie but that's not necessarily the case.
Even when the males are dark you can still distinguish between white crappie and black crappie by the vertical bar pattern on the sides of the white crappie and the random speckling of the black crappie.
If in doubt count the spines in the dorsal fin (big one on top). Officially if there are 5 or 6 dorsal spines it is a white crappie and if there are 7 or 8 dorsal spines it is a black crappie. Personally I've never seen a crappie with 5 or 8 dorsal spines.
We do see a crappie on occasion that looks a lot more like black crappie in color pattern and body build but has larger, fuzzy specks and only 6 dorsal spines and I'm pretty sure they are hybrids - cross between white and black crappie.