"White crappie are distinguished from black crappie not by their coloration, but by the number of hard spines in the dorsal fin. The dorsal fin on the back of the crappie has sharp, stiff spines to the front and softer fin rays to the back. White crappie have five to six dorsal fin spines, and black crappie have seven to eight dorsal fin spines. Be careful not to overlook the first dorsal spine when counting — it often is very short.
Many anglers are misled in identifying black and white crappies by their coloration. If both fish were caught from clear water, the black crappie may tend to have more black markings on its flanks than the white crappie, but color is very unreliable. In turbid water, both fish will be pale with few dark markings."

This is from a biologist.