I believe it can be, but sometimes it just pops up randomly like "OOh, I wanna be FESTIVE today! How bout some pink herruh and some brown there. That's nice! Or wait! How about a pink eye AND a ruby eye, won't that look awesome?"

Kind of like that, but in rat-gene language.

The mother of my odd-eyed boy was also an odd-eye. I have one of Funny Face's sisters and she has a dark brown eye and a dark ruby eye(barely noticeable). So I'd assume it's genetic, maybe like Pastels or Spiders in Ball Pythons. The odd-eye gene just seems to be a dilute to the fur color and the eyes, but since it can happen in black rats too, I really don't know. It would be something to research, I'm sure.