orange tabby turning blonde

Hi there, 

I recently posted the topic "help, aggressive cat" but now I have a new question.

We recently adopted an orange tabby, and I've noticed lately his fur is getting more of a blond and less Orange. How normal is this, or is it my lighting? I don't know if it's due to his diet or not, I know he was overfed previously. He's very docile, non aggressive and non territorial.just wanna make sure my new baby is okay

Thank you 

How old is he?

Hes two. My boyfriend said it might be the change in season because Betsy (our other cat) is a way darker shade of gray