I have veneers on my front teeth, I had a root canal and the tooth turned dark, which made one tooth slightly darker?

I would recommend getting the veneer replaced.  This will completely cover the dark tooth and should match your other teeth perfectly, if done correctly.  I would recommend going back to that same dental office and asking if they can give you a discount on replacing the veneer because of the appearance.  Often times, the dentist will replace it at little or no cost.  I hope this helps and you can follow me if you have more questions or want to read more of my answers.

The short answer and best answer is to replace the veneer. Patching the problem may save money, but the long term solution is to replace the veneer. The sensitivity issue may be resolved by smoothing the resin filling to make sure that there is little or no inflammation of the gum (gingival) tissues caused by the resin. You may want to discuss with your doctor replacing the veneer with a porcelain layered zirconia crown; the zirconia is opaque and masques the discolored tooth. I would do internal bleaching also, because if the gums recede and exposes the tooth surface, you still have the same issue whether there is a veneer or crown placed. Discuss your concerns with your dentist. Most of us really do care about patient comfort and concerns! If you have any more questions or concerns, follow me. Wishing you nothing but the best!