According to genes, eyes can only be one of two colours - blue or brown. Any other colours are just variations on these colours. Variations may appear to occur under certain lights
If we're talking about the iris part of the eye here, I have a feeling that a slight blood pressure in that area may perhaps leach the eye-colour out a little. I've noticed that my own vary slightly between gray and a dirtier grayer-blue colour.
Yes it is true, a lot of people I know have commented that the shade of my eye colour (blue) changes according to my mood. Apparently if I'm angry they go a very deep blue and when I'm happy they are much lighter.