My iPhone cases always turn to a dark color?

My last case was a beige color and after months of use, it was a dark purple. I recently got a white case but that is now a dark pinkish color except for the part around the camera. I haven't been able to find anything about this. Does anyone know why this is happening and how i can possibly revert the colors?

First - start buying black cases. (Joke. Try cases made of other materials.) As to the color change - it is more than likely a chemical reaction between your hands and the case. Could be from a lotion or soap you use. Could be your own body chemistry, perhaps from certain foods. We flute teachers sometimes get kids whose fingers and lips just about dissolve the plating on Instruments. And I have trumpet colleages who have a leather guard over the valve casings - their chemistry eats into the brass. Does jewelry tarnish on you when you wear it? Does the metal button on jeans ITCH your tummy? Rings and watches leave a black mark on skin? Even if you have no nasal or food allergies, chemical contact allergies abound. Even if you are not injured, the chemistry tells you that you are reactive.

No, the colours can't be reverted.

The cause is either or both exposure to ultraviolet light
(the sun is the main source of it)
or chemical breakdown of the plastic of which the case is made.

Start with a dark one and see if it changes at all.