I'd recommend going for greys, blues, red (even yellow) fabrics over black/white. Cheaper black cottons can tend to fade fairly quickly. Cheaper whites can end up a little yellowed without actually staining. Since you've already mentioned that you're limited by licensing costs, I wouldn't be surprised if you end up with slightly lesser materials to fit into the desired price.

The darker grey JayJays has used for some shirts (the Car Voltron shirt comes to mind) is more interesting that the standard grey cotton you often see on shirts, but won't suffer the fading of black cotton.