Are your teeth getting unsightly stains in between? Do you use a mouthwash or toothpaste with cetylpyridinium chloride? STOP!!!!!
This ingredient can cause that weird darkening. I had noticed it on some of my clients, but I just figured they smoked. Then I had a client who doesn't smoke. Did some research. Turns out this ingredient kills the bacteria on your teeth, BUT, if you don't floss and brush very carefully to remove the dead bacteria, they turn black.
Cetylpyridinium chloride is an antibacterial, so you will likely find it in mouthwashes that "don't sting" and some toothpastes.
It appears that the stain is removable by a dental hygienist, although it is stubborn.