It's one of these interesting things from early comics, where they had limited colours, so many heroes got the (art) primary colors, red, blue and/or yellow, many villains (or antiheros like the Hulk) got green, purple and/or orange. Once you see it you can't unnoticed it.

Joker Purple and green. Green goblin purple and green. So on.

Some characters like the thing (orange and blue) or Magneto (red and purple) break that mold, but even then that emphasizes their character's nature. Thing is a good guy who looks monstrous. Magneto is a villain who is often sympathetic.