Hm. Well, the PuTTY icon (which was what sunflowerinrain and I agreed ought to go on a hypothetical PuTTY T-shirt) technically ought to include three different shades of grey/white, two shades of blue, and yellow (plus a black border if the shirt itself isn't black). I suppose one could cut that down to just white, blue and yellow by halftoning, but halftoning would probably cause its own difficulties when it comes to screen-printing.

It's annoying. bjh21 has a dark-coloured T-shirt with what looks like a genuinely full-colour image on the front (gradually varying shades, the whole works), so it's clearly possible to make the things, but I have no idea how.