Not to worry about the DOT5. I've seen the MB Dot4+ turn a greenish tinge also, just like you say, when it's been ignored a few extra years than it should have. No real positive idea why, my guess is something in the system that is bronze, brass, or copper doing a little corroding, just enough to tinge the fluid greenish. Never had a problem with these as far as doing a fluid flush, seemed fine afterwards.

Gilly