Simply make a character with differently coloured eyes. That's ugly enough as it is to me.

(note: no offense to people affected with Heterochromia in real life. RL heterochromia often shows eyes with different shades of the same color, like dark brown/light brown, grey and blue, etc. I'm talking about people who make characters with eyes of completely opposite spectrum colors like black/white, yellow/purple, etc. That's always a punch in the face when I see one cross my screen...)