Color change in cat´s fur

Hi,

I have a 7 months old cat, with black and white fur. He looks like he´s wearing a tuxedo =). Lately I have noticed that his fur changes in color, it seems that the black is becoming a deep, dark reddish brown along his back. He´s an otherwise healthy, playful and happy cat, so I am not really worried, just curious to hear if anyone has experienced this. Could this be because he´s still growing and changing or should I worry and take him to the vet ? 

Thanks in advance, have a nice day.
Tine