The graphic design is intended to be printed on to a t-shirt so the final image can not have a background.
This image may be printed as a small image on the front of the t-shirt as well as a large image on the back of the t-shirt
I would prefer designs with 2-3 colors. Obviously, greater numbers of colors increase the screen-printing costs of t-shirts. Ideally, the primary color would be white so the image can be screen-printed on to a darker colors (black, blue, etc.). However, as an alternative, I would entertain designs with the primary color black that could be screened onto lighter colors (white, tan, sky blue, etc.)