Eye colour apparently does change depending on blood flow through the capillaries in the iris. In rather the same way as mood visibly affects blood flow in one's face, it can also affect blood flow in the iris. The web suggests that this is more likely when you have light-coloured eyes to start with.

There is somewhat better documentation for eye colour changing with age or due to disease, although I imagine the mechanisms there are different (there may be actual pigmentation changes).

(And, IIRC, Polgara had grey eyes under normal circumstances, which somewhat supports the credibility of that particular literary device, although I'm not sure I'd like to claim that David Eddings is particularly knowledgeable in the field of iridology!)