Someone asked, ‘Why are cats’ eyes mostly green?’. The answer is that cats’ eyes are not mostly green, positively not. Domestic cat eyes have a wide range of colours from blue to brown.
There is no indication that green is a predominant colour. The idea that cats’ eyes are green many come from the often green glow from a cat’s eyes at night when a car’s headlights or a torch light reflects off the back of a cat’s eyes as you see in the photographs below.
We know what this is: the reflective layer called the Tapetum lucidum at the back of the eye which intensifies light reception allowing cats to see better at dawn and dusk. The glow from the rear of the eye varies between red-orange to green.
I am not sure of the reason for this color. It’s probably simply to do with the anatomy of the rear of the eye and how the light is reflected.
Green eyes in domestic and feral cats varies from ‘yellowish-green through gooseberry-green to deeper greens and blue-toned greens’ (Sarah Hartwell). Some well-known breeds with green eyes are the Tonkinese with blue-green eyes and the Chinchilla Longhair (Chinchilla Persian) with sea-green eyes.
The link below discusses cat eye color in some depth. The best website for information about cat eye color is messybeast.com by Sarah Hartwell.