This is a response to a question that was almost identical to yours. It is a very informative article so I copied it in full and also provided the link for you.
Teeth can become discolored as a result of exposure to certain "chemicals". These would be stains which would be incorporated into the tooth structure itself; otherwise known as intrinsic stains. Intrinsic stains cannot be cleaned or brushed away. Extrinsic stains, however, are only located as a deposit on the outer surface of the tooth and can be either brushed away or removed with a professional cleaning.
These chemical stains may be caused by metallic and nonmetallic compounds. Iron compounds generally stain teeth black. Iron sulfide can be found as an extrinsic staining material in plaque, and, again, is generally more black in color, although it could contain a greenish hue. Exposure to compounds of copper, antimony, and nickel can impart a green or blue-green stain to teeth. Also, mercurial salts as once used in local antiseptics can stain teeth green, orange, or red. While there is agreement that some fluoride can be transferred to the fetus through the placenta, I am not sure about iron.
Discoloration of the teeth can also occur in children with certain systemic conditions. Certain blood diseases can produce severe jaundice in the newborn baby. One result of this condition is the presence of a yellow to green staining in the primary teeth. Generally, the staining of the teeth, which is primarily green, is not uniform in nature. The middle front teeth may be more stained, while the others surrounding these may be less stained.
Tooth decay can vary somewhat in appearance, although generally it appears in a yellow, grey, brown, or black coloration, and, again, these discolorations will not brush or clean away. In addition, "holes" in the teeth might be noted depending upon the severity of the decay. Normal, healthy tooth structure should be hard to the touch of the dental instrument, whereas decayed areas are soft and sticky.
I agree you should limit sweets and brush as thoroughly as possible. If you wish to have another opinion regarding the oral health of your son, ask for a referral to a pedodontist, call the local dental society for a referral, or check with a local dental school. It is important that your child is carefully examined and the source of the tooth discoloration is determined.
http://parenting.ivillage.com/tp/tphealth/0,,3xfv,00.html
Good luck and best wishes.