I asked an ophthalmologist about this. He said the colour is usually settled by 18 months, but changes can occur up to about age 4 (to darker).
Fair-skinned Caucasian babies (generally meaning of northern European descent) are almost always born with some shade of blue eyes. If the baby's eyes are brown at birth, they will stay dark.
Eye colour is carried on multiple genes so you can have many variations. HTH.