First, your 81 CB750c is DOHC, but also, being that it's an 81 it, like the '81 cb650s, took H4 from stock.
As far as blue goes, I don't know that I have any recommendations. Unless you're doing something very new and avantgarde with this bike, the blue head light might seem a bit out of place, but then the bright white of Sylvania Nighthawk H4s might, too.
so... um. I dunno?