Some of our physical characteristics can be determined by genes expressed together. Eye color fits into this group. Three genes determine the amount of pigment present in the iris, of which, only two out of these three genes are fully understood. They define common eye colors like brown, blue and green.
The first gene defines the amount of melanin in the eyes ( the colour pigment that makes a brown eye colour), thereby determining if brown or blue eye color is expressed. The second gene regulates colors blue and green.
As a rule, genes determining darker colours always dominate those for lighter colors. Hence, brown tends to dominate over green eyes and green eyes over blue eyes. This is why children from the same parents can have a different colour from those of the parents.