The crowns on my front teeth aren't the same color. Can they be improved? (Photo)

It appears that your right central incisor is the main problem with its darkness.  I suspect not only did it have a root canal, but possibly a metal post or else has porcelain-fused-to-metal crown rather than an all ceramic crown.If that is the case, I would recommend that the metal post be removed and replaced with an tooth colored non-metal fiber post.  That won't discolor the underlying tooth.  Then I would recommend that your dentist utilize all-ceramic crowns, possibly utilizing a technique called "the Dickerson ditch" to mask out any discolored underlying tooth with a tooth colored composite that matches your other front teeth. You might even need to have internal or external bleaching to reduce the darkness of the underlying tooth color.Your dentist seems to have done a nice job, however I believe there are solutions out there that would enable a dentist to give you a nicer result.  Obviously you want a nicer looking smile, and I think that can be achieved using some of the newer technques and materials.

Find a dentist who does internal bleaching. Some of the root canal material is removed and a special bleach is used to whiten the inside of the tooth. Teeth that are "dead" sometimes can turn very dark. That is what your seeing through your gum.... A very dark root.