How do people develop a yellow color in their iris?

Sunlight: The iris is the blue or brown part of the eye. The sclera is the white part of the eye surrounding the colored part. The sclera is covered by a thin translucent membrane called the conjunctiva. Often, harmless yellow growths called pinguecula develop (due to sun exposure) on the conjunctiva at the 3 and 9 o'clock position on the sclera adjacent to the iris. See your eye doctor to be sure.

DNA: For the most part, eye color is determined by genetics. There are very rare conditions and some glaucoma eye drop medications that can change iris color, but yellow is most likely a genetic variant where there are a mix of dark and light genes in combination.