Cats' eyes sometimes darken probably because they are undergoing a natural visual adjustment process. This usually happens when the cat changes from a dark to a light environment or vice versa. In this case, your cat's eyes will become dimmer until they adjust to the new light levels.
However, if your cat's eyes have been dim and this has lasted for a long time, Then it could be due to other reasons. Some diseases, such as feline cataracts, can cause cats' eyes to darken. There are also some medications that can cause darkening of the eyes. If you notice that your cat's eyes are consistently dark, you should take him to the vet.
