Eye colour in wild animal species is generally fixed. However, humans and domesticated animals show various eye colours. Why is this so? BTW, when we say eye color, we mean the iris which is encircling the pupil. The pupil is always dark and dilates and contracts quickly depending on conditions of surrounding light.
It has been noticed that eye color variants for humans started quite recently (approx. 8000 years ago). As of today, eye colour variety in humans could be described as continuous, with large number of shades from light blues to very dark browns.
In wild animals that have no variation in eye color, it looks like this trait is adaptive and fixed by nature selection. It is possible that evolutionary processes would be at work regarding eye colors: domestic animals artificial selection, animal species natural selection, and maybe sexual selection when humans became farmers.
conceived by @stalexmkl