It's probably a dead tooth or almost completely rotted. Gray really isn't normal for a tooth. Also, right here doesn't explain where it is....

Are you rough when you brush your teeth? I remember hearing somewhere that if you have grey on your teeth, it could mean you've damaged the enamel. Would be a good idea to get it checked :-)

Thanks for the input you guys gave me.
Sorry for not being more accurate in the details...
Its not gray, from outside... It looks like its from the inside... So that takes out coffee or cigar stains.
I googled for the name and its my second premolar tooth that got gray.
Now its not hurting when I drink cold drinks because I changed my teeth paste, but if I push the tooth I little hard and let it go, it hurts a bit. Its not moving, though... As firm as the others. And still the same grayish color
three other things:
first, I smoke... Could it be related?
Second, when I floss it sometimes it 'cuts' the dental floss... Maybe a crack on the sides of it? Decay?
Third, grinding? Sometimes I get myself at night grinding my teeth... Not that freequent tough.
Really dont know what to do.. Im really tight on $ so im trying to find if its really necessary to go to the dentist or not.