From what I can tell, Engine Ice is monopropylene glycol with a small amount of a couple additives that are probably corrosion inhibitors. It's chemically similar to ethylene glycol, and mixes well. Ethylene glycol is a little better a transferring heat then propylene glycol, but it's also more toxic, in case you plan to drink it. Independent sources confirm this, but I've seen company literature saying not to mix them. I suspect that's a self-serving warning.
As far as Spencer's questions, not mixing different colors is certainly a safe plan. Personally, I hate marketing tactics that are designed primarily to get more money from me, so I'll mix anything I want in my motorcycle(s), and if I have any odd reactions I'll report them here.
