White shirts would be a great help for ya...I would pick out all the colors I could halftone or mix to make other colors. Maybe Dan can chim in and help you he has a very keen eye for color mix and seps. I going to toss this out there, on a black tee you might can print some top colors on the white base wet on wet to give you a more pastel blend of colors, then flash and print your more bright colors if this makes any sense
Darryl