Black on Gray, Light colors tend to look "dingy" fairly quickly unless they get bleached frequently and bleaching wears out the shirt quicker. I have some grey ATC polypro tees that have about 1000 miles of backpacking on them and they still are somewhat presentable to mixed company. I also strongly would like polypro and would suggest Duofold for the source of shirts if you have a choice. They are usually reasonably priced and seem to hold up pretty well.
Typically the screen printer can mix base colors of the shirts quite easily, what is more difficult and general costs a bit is to change ink colors. Therefore pick the ink that works best, which may be green, as black on white is less attractive then green on white.
An option may be to order two base colors, and offer them first come first served, that will hopefully gets folks motivated to get them on order quickly so the process doesnt drag out and the stragglers will be glad they got a shirt no matter what the color.
