Best option for darkening tooth?

I think that you are headed in the right direction with the treatment that you have in the works.  Following the root canal, I would place an all porcelain crown or veneer on that tooth to correct the coloring.  Depending on the case and alignment of your teeth, you may need to crown the tooth next to it.  Best of luck. 

Getting one crown/veneer to match all the rest of your natural teeth is a very difficult thing to do--probably one of the most difficult things in dentistry.  However, it is possible.  Make sure your root canal is being done by an endodontist.  For your crown/veneer, visit a dentist that is very experienced with cosmetics.  Don't let the permanent restoration be seated until you have approved it yourself.  An all porcelain crown/veneer can definitely be made to match the rest of your teeth, it just takes an experienced dentist and technician.

The best solution for your situation is a rooth canal for the dead pulp tissue and a porcelain veneer. The advantage of porcelain veneer is that you can make it with the same or similar color of your teeths. 

discoloration is because of dead pulp tissue due to trauma. ideal treatment option is root canal therapy with pulp  whitening . but sometimes it is not possible to achieve ideal results , in that best option is veneer. 

When your dentist performs the root canal procedure, they can do an internal bleaching procedure using a bleaching product like Opalescence Xtra. This will rid some of the discoloration. In addition, you can also do an external bleaching procedure with in-office or take home whitening. If the tooth still appears darker, then you will have to proceed with a veneer or crown. The shade of the veneer/crown can be matched pretty closely if shade matching is done correctly by the dentist. We generally send pre-operative photos of your teeth to the dental lab that will be fabricating the veneer/crown to mimic the shape/shade of the tooth perfectly.