If the cat is solid white, its fur could be stained by kitty litter. Keep the box IMMACULATELY clean.
Saliva can stain the fur, too, as it oxidizes on the fur. Is the cat licking a lot more than normal? I know I see a change from white to yellowish in my cat's hair whenever he grooms excesively, which can happen a lot, especially due to anxiety, boredom, or just simple pleasure.
Bluing shampoos may return a pure white coat to its show-quality colour. Many Persian fanciers prepare a cat for show by shampooing with dish soap to remove everything from the coat. Add a bit of laundry bluing to make the cat sparkle white. Just not too much or you'll have a blue cat.
I would need to know quite a bit more information. "How old is the cat" is a good question to start with.
Also, especially if the cat isn't especially old, are there any other symptoms? Is there anything going on with the skin or does kitty show any signs of not feeling well? Any other changes at all whether behavioral or physical?
Let me know and I will see if I can narrow down some possibilities. However, if the cat is young and particularly if there are any other indications of abnormality, you might really want to schedule a vet visit to figure it out.
hmm- how old is the cat? I have a tuxedo who is 5 and has a few changing to grey- could be just like a human who greys as they get older? Anyone else have thoughts? I could very well be wrong.