What ever it's goign to be it absolutely must be white so that everything shows up. When we had our mixer we didn't have a t-shirt made up for it...just went to Target and got some white fruit of the Loom shirts. just making everyone go for their own shirt is much cheaper than having shirts printed up.
IMHO, I would go against making shirts for the event b/c they aren't going to end up being something you'll ever want to wear again outside, so it seems
VERY, VERY pointless to spend 10 or 12 bucks a shirt for something that is only going to be worn once due to the crude remarks. Far better to spend $5 and get three shirts of which 2 can be used like normal. So that's something to think about.
In all honesty I can't think of one good reason why you would need to print up shirts, but that's just me...
Either way the party will be a blast, so have fun...