So I may sound like an idiot but if the acid that scared Harvey dents face was strong enough to burn him you’d think that he should have no hair and even if it grew back it wouldn’t be another color right
And even though it was half his face
I think you’ve stumbled on a major revelation here: Two-Face uses multicolored hair plugs.
Maybe it scarred his face, but it didn’t touch his head. (What luck, right?) But the massive fear and derangment made his hair change.
It wasn’t perfectly split down the middle. I think HCQ is right, Two-Face’s insanity urges him to use product.
Another possible explanation – Harvey was suffering early pattern baldness and wore a toupee, and the acid messed with the color of the wig.