There are a few reasons why your cat's fur is turning gray. First, age is a factor, like humans felines will gray. Second, gray hairs do not show as visibly when there is a mixture of white and black because they sometimes overshadow the gray. You did not mention his/her age would have been helpful.
Felines hair does lighten a bit as they get older, just like our hair. it is not something to be concerned about, as long as the fur still looks healthy. if the cat starts losing hair, call your vet immediately. otherwise , your cat is getting older and turning "grey" just like we do.
Some animals, like humans turn gray as they age. In cats however, gray hairs can sometimes mean the cat has a build up of dead hairs caught in it's fur. You can fix this by brushing your cats fur regularly to help release shed hair.