Front tooth had trauma about 12 years ago. Had plastic bonding done, now my real tooth is discolored. (Photo)

See a good dentist to take an xray to check for pathology, since you mentioned sensitivity on your single front tooth.If tooth needs root canal , after root canal, have the great dentist do the internal bleaching, right after root canal, you will see your tooth will look as original in 3 to 4 days. If you do not need root canal after the xray, here are your options:1- Bleaching and tooth whitening on single tooth or all your teeth, then change the dental bonding you had many years ago after bleaching and tooth whitening.2- Have a complete dental bonding done, basically it is a composite veneer, It can stain on the long term, and it is not as strong as porcelain veneers, however very little tooth structure is removed, which is less invasive than porcelain veneering. Cost is moderate.3- Depending on your bite, possibly you might need a porcelain veneer. Porcelain veneer is strong, however tooth preparation more than dental bonding is required, and it is a 2 office visit to the dentist. If you do choose to do porcelain veneer, for a lasting result, I recommend you find the best cosmetic dentist near you, since to do a one tooth veneer requires skill, vision, a great master ceramist. But again this is for lasting results, and even if the tooth goes dead in the future your dental veneer will not be damaged. the cost on porecelain veneer for single tooth can be expensive.

A dark tooth is common after trauma. You need x-ray and endodontic evaluation to eliminate pathology. If you need root canal treatment, get it done first, followed by internal bleaching and a crown. Otherwise, get a porcelain crown. Bleaching of the tooth before doing the crown might even the color. 

an x ray evaluation will give the decision, about the protocol that you need, there is a thing to consider, that is the length of the root, and if the apical formation of the tooth was interrupted, the sensitivity is normal, due to a hyperemia, and the discoloration is normal, the only way to recover this is with performing a root canal, post and crown. bondings would not work