The answer to the question in the title is found in genetics, cat anatomy, kitten development and physics. When kittens are born they are not fully developed. Some development takes place after birth. One aspect of development that takes place after birth is eye color. The adult eye color, the final color, is coded by the cat’s genes.
Eye color is due to:
- the concentration levels of a substance called melanin in the iris of the eye. Melanin is a dark brown pigment that is also in the cat’s fur. It dictates the color of the fur and the pattern in the fur.
- the way light passes through the eye and is scattered in the eye.
Cells in the cats body produce the melanin. At birth there is not much melanin in the kitten’s iris.
Why does a small amount of dark brown pigment in the iris of an eye make the eye look blue? Answer that question and you have the answer to why kittens have blue eyes.
The physics is complicated. Light of a longer wavelength is red, orange and yellow. Shorter wavelength light is blue. In summary, light of a longer wavelength is absorbed by the pigment. This stops the colors being seen.
Light of a shorter wavelength is scattered in the iris and is visible. The scattering of the light is the same as the scattering that takes place in the sky. In both instances the light looks blue in color.
Accordingly, the blue of blue eyes is not due to blue colored pigmentation in the iris but is caused by a physical phenomenon.
In due course the cat’s eyes settle on the adult color. The most common color is from greenish-yellow to gold.
Siamese cats have blue eyes because a gene, causing a form of albinism, that removes color from the fur and from the iris.
Cats with some white fur – bicolor cats – sometimes have blue eyes. The white fur is coded by the piebald gene which also affects the eyes.
All white cats have blue eyes due to a lack of pigmentation. The fur is also white due to a lack of pigmentation.
Sometimes the piebald gene causes on eye to lack pigment while having no effect on the other eye. This results in odd-eye color. The gene that produces an all white cat can have the same effect. The famous all white Turkish Angora is most popular when it has odd-eye color, one eye being blue (lacking pigmentation), the other usually yellow/gold.
A significant percentage of white cats are deaf. The deafness is caused by the same genes that cause white fur. Deafness in white cats is a result of a “lack of pigment on internal ear structures”.