There is a reason coolant is colored, from green, red, pink, blue, purple is to distinguished the main chemical base. So when it is mixed, the longevity may not meet designated standard performance. Same like engine oil, all can claim API SN and can be mixed, but the chemical composition may vary for each brand, and when people mix product they may end up with inferior products. I hate my local Subaru dealer just because they are ignorance enough to top up coolant on reservoir with water. Of course based on their servicing package, this coolant will be replaced before it get rusty, but for peace of mind on simple stuff that is supposed extremely low maintenance, I will just use Subaru coolant to top up.