The majority of ponies have dark brown eyes. In some light the entire eye can appear to be black, but the iris is dark brown.
Pinto Blue Eyes
Ponies with bright blue eyes are a result of them having a pinto pattern/white pattern. No independent blue eyed gene, such as in people, has been identified in horses. Pinto blue eyes are brighter than the blue eyes found in double cream dilutes. Ponies could have two blue eyes, one blue eye, or partial blue eyes.
The Splash White pinto pattern (SW1) is generally the reason for blue eyes in Chincoteague Ponies. Homozygous splashes will usually have two blue eyes. Heterozygous splashes can have two blue eyes, one blue eye, a partial blue eye, or none at all. The inconsistency of blue eyes in ponies with splash white is why blue eyes sometimes seem to skip generations or seemingly appear out of nowhere in a family line.
The Tobiano pinto pattern that is so common in Chincoteagues has not been found to cause blue eyes. Occasionally Chincoteagues will not visually appear to have a pinto pattern other than tobiano, but blue eyes indicate the pony has another pinto pattern. Splashed white, specifically splashed white 1, is the most common cause of blue eyes in Chincoteagues.
Other pinto patterns such as Frame, Sabino1, and White Spotting can have blue eyes. Frame and Sabino1 are not found in Chincoteagues and the presence or frequency of any of the White Spotting patterns has not yet been proven. There are also likely untestable versions of splash accounting for more occurrences of blue eyes.
Foal Blue Eyes
Like many newborn animals, some foals will be born with blue eyes that changes to brown when they get older. They are a different shade of blue than double cream or pinto blue eyes.
White Sclera, also called wild eye or human yee, is the white area around the colored iris of the eye. All ponies have white sclera, but it is usually not visible. The white sclera on some ponies is always visible and is due to the shape of the eye. On others it is only visible when the pony has moved its eyes.