Dunno about the shirt color-- they're about impossible to mock-up accurately on screen. You might have to lay hands on a shirt.
As far as art goes, screenprinters vary wildly in quality and capabilities. I'd talk to the specific printer about their specs, but here's my general advice:
Line-art = Illustrator, or some other vector program. Don't set trap or overprinting on anything. Leave that up to the printer. I get about 1/2 a point of spread when I print a shirt, but I use tight screens and sharp squeegees. Bad printers who use loose mesh, or too coarse mesh will get 1-2 points of spread, so keep that in mind when you set your fonts. (you're going to lose serifs on a 10 point Times-- if you've got to go that small, stick to non-serif fonts) Covert all fonts to outlines before sending the job out, to avoid conflicts.
4c process: avoid unless you image is continuous tone/photographic. Most screen printers will tell you that you can't reproduce anything finer than 55 line screen on a t-shirt. This is malarky. (I print 85 line all the time, but 65 is industry standard)
Unfortunately, most t-shirt printers are too lazy to figure out how to print any finer. I'd quiz the printer-- ask them how they avoid screen moire, what percentage of dot gain to expect, and what tonal range they can hold. If they can't answer any of these questions, find another printer.
Photoshop image sizes, figure 2.5 times the screen ruling, at 100% print size. If you're reproducing line-art in Photoshop, (which I don't reccommend) make it 300 dpi at 100% print size.
Good luck,
CV