Ok, in my school I'm part of the Astronomy Club, (yes I know I'm geeky), but anyway we had a little competion for desgins for t-shirts, mine won and now I have to tone down the colors so it can be printed, I don't know any specifcs (number of colors, max resolution, etc.). But I now theres a way to have Photoshop do some of this easily (resolution is a no brainer), but also is there a way to get it to limit the number of colors used in the pictures, or anyideas how to do this without looseing my desgins too much?

heres the pic:



Thanks for any ideas in advance.