What causes peoples eye color to change?

A few conditions. Generally, eye color becomes "locked in" by about 6 months to 1 year of age. The iris contains brown pigment, and the less pigment present, the lighter the eye color becomes. Diseases that damage the iris (inflammation, certain glaucomas...) can lighten it while tumors and freckles can darken parts. High corneal cholesterol can make the eye seem bluer. Any changes in eye color should be evaluated.
Eye color. Some children with blue or hazel eyes can have some darkening with variable amounts of brown as they age. If there are similar changes in both eyes, it is usually normal. If one darkens and the other doesn't, that should be checked.