Well, to kinda shade the black shirt, I suggest finding a somewhat dark shade of grey, then finding another shade of grey that's darker than that, then black. If you look at Cynthia's clothes, you can see that, even though they could be considered black, it's really a dark shade of grey. Looking around in everyday-life, shirts that you consider black may look somewhat grey-ish in the sunlight, if you know what I mean.
Hope I'm making sense. If not, you can PM me, and I'll try to explain it a bit more.
For now, I suggest that, instead of finding your own colors, try using some existing colors already found on the other Pokemon and trainer sprites from the game.