I'm working on a design right now, I'm just wondering if the colour of the shirt counts as one of your 6 colours. Also what constitutes a dark shirt? Also when your using halftone dots to get different shades are there any restrictions such as dot size, contrast, or softness of the dots?
darker shirts range from the richer-colored tees (red, green, turquoise), to the typical browns, charcoal, and black.
halftone sizes can vary as long as they aren't *too* tiny. if you were to print out the image on a sheet of paper and couldn't easily see the differentiation between the dots and negative space, it's probably too tiny.
gradients are tricky, i think. in most cases, they would probably just be turned into half tones, but i think avoidance of trickier print processes usually gives a design better chances at being printed.