What causes the coolant color to turn milky?

There is only one thing that comes to mind on your problem of a milky coolant and it isn't good news. I am leaning towards you having a cracked block, a cracked head or a blown head gasket. What it sounds like is that engine oil is mixing with your coolant. There are only a few places where this can take place, which I have mentioned. I would run a pressure check on your colling system and watch for the pressure to slowly go down. Also look for milky oil contamination. I wish you the best of luck.