This is a photograph of an all-white Maine Coon cat with heterochromia iridum. In layperson’s language, this beautiful cat has odd-eye colour. And it is a classic case of odd-eye colour because one eye is yellow-gold and the other is blue. The reason why one eye is yellow-gold and the other blue is because of the dominant white gene (WD) which has removed pigmentation from the hair strands and has removed pigmentation from the iris of the left eye leaving it blue. It is blue because of the refraction of white light through the cornea. We see the refraction of light creating blue light in the sky. This is because small particles in the sky refracts white light into a spectrum and we see blue as a dominant colour in the refraction process because it is scattered the most. The same process occurs in the eye. In addition to the dominant white gene removing melanin, otherwise known as eumelanin from one eye, it is said that a lack of genetic biodiversity contributes to this condition.
Eye colour is created by the amounts of two pigments present in the iris of the eye: eumelanin and pheomelanin. The former is dark brown or black and the latter is red-yellow in colour. Therefore in the odd-eyed cat on this page the yellow-gold colour of the right eye is predominantly created by the pigment pheomelanin in the iris of that eye. The iris is the circular sphincter muscle which creates the variable-sized pupil through which light passes to the retina.
The word “heterochromia” in relation to cats always refers to eye colour but can also apply to colour variations of hair or skin. In addition to two eyes of different colour, you can sometimes see a shift in colour within one iris. This is called “sectoral heterochromia”. You can see a picture of this in a beautiful random bred cat on this page below. A further variation is “central heterochromia” in which there is a ring or spikes around the pupil of different colours radiating outwards.
In the top photograph you see a Maine Coon in the foreground and another all-white cat in background which is not, in my opinion, a Maine Coon. Or the appearance is not that of a Maine Coon because the muzzle is too pointed and not square enough and the ears are not lynx tipped i.e. hair tufts coming out of the tip of the ear flaps. The cat may be a Maine Coon which does not fully meet the breed standard.