Where do you draw the "classic" line?

I feel red/green/black was the most common scheme in the horror I saw as a kid ('70s-'80s), tho perhaps it was purple and not black..

Classic Horror generally means into the early 1950s. The genre changed a lot after that. So yeah, 70s/80s horror aesthetics were quite different.

Isn't '30s to '50s usually get called the Golden Age, with Classic being until the late '80s?

Huh, that's a new one to me. If some people are using it that way, I need to be careful about using "classic". In my reading, both Classic and Golden Age have been used to mean the era dominated by Universal Studios.