My tooth is turning grey?!

I just noticed that my one of my molars is turning grey. It's not completely grey. Half is a pale grey and the other half is normal.I very worried because I know that might mean my tooth is dying. I really don't want to have to get a root canal, I'm 14 !! I brush twice a day and floss. I'm not in any type of sport so I couldn't have traumatized it. I do wear retainer because I use to have braces. What's happening to my tooth!