To clear misconceptions in regards to changes in eye color, the distance between the eye and object of observation cause changes in the size of the pupil; when this happens the pigments in the iris either compress or spread out resulting in a potential minor change in observable color. I presume that this can also be caused by a variety of other triggers such as moods and emotions or even states of mind.
Another major factor in determining color is obviously the lighting in which it is observed at a point in time. Depending on genetic factors, a natural change sometimes takes place in the initial stages of development and this is directly linked with the quantity of melanin one posesses and also applies to skin pigmentation and hair color, in almost all cases increasing with age (up to maturity) and thus causing a darkening of pigmentation.
