Heterochromia

It's a condition which displays a difference in coloration, usually of the eye. And it's not just pets, but people. For example:

Kate Bosworth

brown (right) & blue (left)

Jane Seymour

brown (right) & green (left)

Mischa Barton?

gray (right) & blue (left)

Not so fast! Whether it's a trick of the light or just a trick, period, another shot from the same session shows Mischa matches.