They should be green in the Overworld, orange in the Nether and purple in the End and within 200 blocks of an end portal.

It shouldn't be too hard to implement, considering how an endermen's eyes are already a seperate texture on their own.

An additional but probably unnecessary addition could also be that if you push a green or purple eyed enderman into a Nether portal, or a Nether enderman into the Overworld, it should go into "rage mode" and start attacking whatever creatures it can find (on account of it being forced to change dimension unwillingly)