Why color of peoples' eyes can change?

I think it is due to the pigment present in the iris. I have never heard about such thing, that the colour of the iris could change on effects of emotions; as far as I know the colour of the iris is given by the pigment present. Typically, newborns have blue eyes, which change to green, light blue, light brown and dark brown, as they grow older. It is thought that exposure to light after birth triggers the production of melanin in the iris of the eye. By three years of age, the eyes produce and store enough melanin to have their natural shade. While changes in eye color of infants are more common, even in adults, eye color changes in approximately 12% of the people as they get older.

more information on the colour of human irises, go to:
iris (anatomy) or eye color @ wikipedia