Light eyes have very little pigment, whereas darker eyes have a lot. In newborns, the pigmentation process of the iris is not yet complete. Babies with darker skin are usually born with dark eyes that stay relatively dark. Iris color in lighter-skinned babies is usually a blue or bluish-gray color at birth, then change as they grow. Melanin production changes during the first year of life, usually resulting in a darker, deeper eye color.
Permanent eye color is not set until a baby is at least nine months old, so wait until your child's first birthday to determine what color they will be. An infant's eye color is influenced by the eye color of their parents. Eye color is often studied in the field of genetics because of its inheritance patterns, but is still not fully understood.
Source: Alfred Rosenbloom, Jr. and Meredith W. Morgan, Principles and Practice of Pediatric Optometry. J.B. Lippincott Company, 0-397-50917-0, 1990.