Here's a picture (also swiped from FB, ha) of my chapter's second invite dresses.
You can't really see too many of the dress, but you get the idea.
There were three dress colors, light blue, dark blue and gold (although there kind of ended up being a lot of colors because some of the dark blues were navy while others were royal and some of the light blues were more turquoise and others were more powder blue), but I think the variety ended up looking really nice.
As far as how you make sure that there's an even number of colors...we were assigned dress colors. Our recruitment VP notified us what color we were suppose to wear during the spring before recruitment so we could have plenty of time to look for one. Be careful with gold though, it may not work with all skin tones, so you may want to give special consideration when you're deciding who wears gold.