How old is your cat and how old was he when you just got him?
It is quite normal for cat to change the color of their fur as they grow up or get older. Some people also say their cats' color can change according to the amount of sunlight they get. While others say their diet have an impact on the fur's color too.
Fishbone is also right. Some cats can get lighter colored fur in the summer. I guess this one is in fact related to the amount of sunlight they receive, since we get longer day time during the summer and naturally more sunlight.
