Why does the tooth structure become black and sometime become yellow when there are dental caries?

Is black, it's worse: Dental caries do not cause the tooth to become black. That usually means the dental root or nerve has been destroyed. Get ye to the dentist - now.

Carious dentin: The dark shadow that can appear under the enamel when the tooth is wet is decayed dentin. The tooths surface may even look intact. The yellow under the enamel would most likely be healthy dentin.

Infected: Dental decay is bacterial infection of the various tissues that make up normal tooth anatomy. Some bacteria produce discoloring pigments. Some destroy tooth material. Don't wait, see your Dentist now before your faced with systemic spread of the infection.

Demineralization: Bacteria like Streptococcus mutans produces acids that leach the minerals out of our teeth causing soft areas called caries. The demineralized tooth can pick up many colors. Off white caries are usually faster moving, causing fast destruction. Black caries are usually slower moving but may have faster moving caries underneath.

Tooth Decay: The cavity in the tooth catches food products, bacteria, and sometimes the tooth itself attempts to repair/slow down the decay by laying down tertiary, sclerotic dentin. Any of these can cause discoloration of the teeth.

See your dentist !!: When dental decay is visible to the naked eye the decay has traversed the tooth and a visit to the dentist is needed. Prior to actually visualizing the black or yellow decay, a radiograph and or visual examination by the dentist can pick up incipient lesions.

Destruction of: enamal creates the color change. See your dentist asap.

Demineralization: The acid demineralizes the hard structure of the tooth and the extent of damaged tooth structure became evident. Cavities on the front teeth are the easiest to see and will look like a yellow-brown or black spot.