A cat that has two different eye colors has a feline version of heterochromia iridis. Cats that have heterochromia iridis are also called "odd-eyed cats" and this usually affects white cats or those with the white spotting gene.
Heterochromia iridis is Greek for "different colored irises and is not a medical condition but a name given to a physical feature, unless the cat's eyes change color when they are already mature. They are often thought to be deaf in one ear; however, this is a common misconception. White cats with one or two blue eyes do, however, have a greater chance of becoming deaf.
It is caused by the white gene or white spotting gene prevents the cat's eyes from receiving melanin while it is developing in the first 12 weeks after being born. All cats are born with blue eyes, but if no melanin reaches the iris as it is developing, the eye will stay blue. The color of the eye in the 12th week is the color the eyes will be throughout adulthood. The genes that make the cats white stops melanin can sometimes stop only one eye from receiving melanin and means that one eye will be blue while the other can have another color.
If an adult cats eyes start to develop different colors, this is something that should be inspected by a veterinary surgeon. It can be caused by iron deposits or inflammation. If a cat's eyes start to become lighter, it could be uveitis.