Here's what looks like a fairly straight article about antifreeze:
http://www.baldwinfilter.com/literat...9;s/05-2R1.pdf
It looks like old fashioned ethylene glycol antifreeze is green, and other colors are now used almost randomly for the other types of antifreeze. It also looks like there is little harm in mixing different types, but to stay with particular properties you might want, obviously you should stick with antifreeze that gives you those properties.
There are apparently some problems with GM's Dex-Cool antifreeze, resulting from its tendency to soften some plastics, and possibly damage gaskets. There's some sort of class action suit in progress related to this.