Would tooth decay be fully black?

Maybe.: Decay comes in many colors, and so do teeth without decay. Hardness is a better measure for decay than color, and for a specific answer see your dentist. Things to answer before you call: do you have pain? Do you like your smile? Has the color changed? How long has it been since your last cleaning? Good luck!

Not necessarilly: Some badly decayed teeth may turn black due to an old amalgam restoration. Decayed teeth come in all colors.

No: Teeth gradually become darker as they decay more and more.

Not necessarilly: Decay has many 'shades from pale yellow to brown to black. In a young tooth with rapidly formed decay, it may be somewhat yellow and is has not had time to stain. Conversely a black area on the tooth might just be stain!

Not always: The longer the tooth is not fixed the darker it can get. Sometimes the decay never changes color, it all depends on your diet, the meds you are taking and every individual is different. Get them fixed as soon as you notice a decayed area. Your dentist should be finding them for you way before you ever notice them.

No: Tooth decay can be black brown or even white.

No: Decay can be black, it can also be brown, yellow or orange.