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.