After making about 100 shirts (ok closer to 50) in the last few weeks, here's what I've learned:
1. matching shirts for your whole party is cute...go w/color that folks agree on.
2. pull out a design element and go w/that for color determination.
Here is what we did (for a group of 8):
Epcot design which was mostly gray, we used orange shirts to put them on since that was the color everyone liked at Old Navy...LOL
Seven Dwarfs designs by LadyLucca which are industrial looking, we used ash color shirts to keep w/the industrial look.
WDW sign w/family names and clipart, we used blue shirts since lettering had some blue in it (contrast most of the color of the rest of the design).
Mickey TieDye done w/white shirts
Kids designs (kids made their own iron-on designs once they got into it) were done on NATURAL color shirts...these are my favorite as they are not white, but can use the white/light iron-ons (wish I bought more).
Spray mickey dye heads done on white shirts, then sprayed w/glow in the dark also so mickey swirls will glow on the rides.