In general, it's a bad idea (or merely pointless) to mix different antifreeze colors or formulas together. At the very least, mixing antifreeze formulas together would cancel out the benefits that a particular solution might provide. For example, OAT or HOAT-type antifreeze solutions have numerous benefits such as inhibiting rust in a car's cooling system. Thus, adding IAT-type antifreeze solutions into either an OAT or HOAT mix would essentially cancel those advantages out, and would bring no benefit to either one. And, in some rare but extreme worst-case scenarios, the two different antifreeze solutions might even gel inside your radiator, which isn't good.