If it happened pretty rapidly (like overnight or over a couple of days), your snake is probably going into shed. I have a normal female right now who is very dull. Her eyes haven't gone blue yet, but I can see all her color drained from her.

That said, if she's not in shed, there could be a lot of reasons she's losing color. A picture would be really helpful. Also, poor nutrition can dull out a snake. How often are you feeding the snake and what are you feeding it?