Lutino cockatiel changing feather color

(Savannah GA)

I have a lutino cockatiel who is about a year and a half old. When I got him I was told by the breeder he was male but I have always believed him to be female even though I still call him a boy. Anyway,my question is about his feathers.

Over time his feathers have started changing colors. When he was younger his tail feathers looked dirty at the ends mainly on his tail feathers. It is a dirty brownish color and has never come off. Over time it has become more apparent and is also now on the ends of his wing feathers as well. It is darker and more noticeable. Any idea of why it is like this? Thank you.