Green as a representation of evil

I think The Saucepan Man hit the point.

...it’s more likely to provoke a bad reaction in situations where we don’t expect it.

Green leaves or green grass are ok, but green fumes are not. And like mark12_30 says, it's the same with any color.

Black clothes are fine, but the all-black darkness of the black riders is something odd and scaring.

And Saruman's white is not a real white any longer. It's made from all colors, and it's some kind of the optical illusion. Very unusual at the Third Age, a long long time ago...

It surely was peculiar to Sir Gawain to meet the Green Knight. But the Red Knight or the Pink Knight would have been the same thing. All the real medieval knights were actually rather gray, I think. The synthetic colors was made up much more later. And before that there was not a way to color brightly.

Sauron, of course, was a master to demonstrate his might by producing the things unnatural.