http://www.planetcatfish.com/common/species.php?species_id=52 : Ameiurus nebulosus (brown) is similar to Ameiurus melas (back) but has 5-8 large saw like teeth on rear of pectoral spine.
The colors are too variable to distinguish blacks, browns, and yellows. Perhaps when you have 60 of them side by side, all from the same body of water, you can spot the difference, MDFishTanks?
From what I can tell, the black bullheads have full black/gray barbels and the browns barbels start lighter, almost white and fade in to a darker color. When I look at them all swimming they are many colors - med brown, black, silver, slate gray, some have blotches, etc.
Same with the tilapia - one day they are almost white and the next they are sky blue and if I take them out of the water they turn to a really dark gray/chromatic silver color.