Teeth Whitening for Grey Tooth from Injury?

Attempting to whiten your front two grey teeth would be a mistake.  The stains present on these teeth are deep within the teeth and whitening is only effective on surface stains. The only viable solution cosmetically is to place porcelain veneers over these teeth.  This would give you a great result and should either of these teeth ever need root canals they can be accessed from the inside of the tooth leaving your veneer untouched.  

It is possible for your tooth to lighten with tooth whitening, but the result can be unpredictable. A tooth will darken following trauma because of hemorrhage in the center part of the tooth. This causes the pigment heme to get into the tooth structure from the inside.
An x-ray can't tell you if a tooth needs a root canal; it will only tell you if the tooth has a low grade infection that has caused bone loos around the root. Very often, the nerve will slowly die in teeth that have been severely traumatized.
You should have your dentist perform a pulp vitality test to see if the nerve is still healthy. If the nerve is not healthy, then the most predictable way to lighten these teeth is to do a root canal and then bleach the tooth from both the inside and the outside. If the teeth get lighter but still don't match, then a porcelain veneer may ultimately be needed. However, it will be much easier to get a great result with a slightly darker tooth than it will be with a significantly darker tooth.

Trauma to your front teeth that has caused them to turn grey is the hardest color to whiten. Usually when teeth appear grey, they are usually darker (grey) even deeper through the tooth. Meaning that whitening can even make the grey stand out more because it causes the enamel to become more translucent. The gold standard for grey coloration of your teeth is going to be porcelain veneers. I would also recommend going with feldspathic type as well (not pressed like Empress). Ask your dentist that you would like to have feldspathic veneers made. In addition, to completely mask the grey coloration- when they go to cement the veneers ask your dentist if he/she has any pink cement to mix in to the traditionally cement. It'll turn "salmon" in color but this has the best chance to 'neutralize' the grey color. I know it sounds crazy, but this advice alone will save you back and forth trips to the dentist for shading. Hope this helps.