Funny you should say that right now. I had just this moment said to drswirly that there was a David Eddings character (Polgara) with that somewhat implausible characteristic, but that she was magically enhanced and hence was probably allowed to cheat. I'd taken it for granted that that one was definitely exaggeration and couldn't happen in the real world! How odd.
On the contrary.. depend on levels of illness and anger, Tim's eyes go from aboslutely colourless mid-grey to VIVID blue. I've seen mild changes in the past, but his are quite startling. I could give you approximate hex values as ilustration if you want... ;)
Mine are normally vaguely hazel, but I know they do go greener. I believe it's to do with blood flow, personally; if you look REALLY CLOSELY at an iris, it looks like it's made of lots of little folds; in my case, they're speckly greeny browny rust-red. As they change in size (well, pupillary constriction) and so on, they will have different bits of colour to the fore. Well, it makes sense to me anyway.
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!)
romauld has grey-blue eyes that go from pale blue to dark grey to green depending on levels of arousal and anger - very simplistically it relates to how "hot" or "cold" he is feeling. It is interesting.
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Mine are normally vaguely hazel, but I know they do go greener. I believe it's to do with blood flow, personally; if you look REALLY CLOSELY at an iris, it looks like it's made of lots of little folds; in my case, they're speckly greeny browny rust-red. As they change in size (well, pupillary constriction) and so on, they will have different bits of colour to the fore. Well, it makes sense to me anyway.
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).