How to lighten gray appearance of gum tissue that has had root canal therapy with post placement? (photo)

Hi:
Unfortunately as long as the root canal tooth is there, any fix will be temporary.  Ultimately, a zirconium implant with a zirconium abutment may be the only way to get rid of the gray.
Best,
Dr. Maddahi

Brytieboo!
Managing anterior tooth aesthetics following root canal therapy can be a very daunting task for any cosmetic dentist. A non-vital (dead) tooth slowly discolors over the course of many years immediately following the root canal. It appears that you have already started the whitening process by undergoing internal bleach therapy where the dentist places carbamide peroxide within the canal & replaces the bleaching solution until we have achieved the desirable shade. However, it is contraindicated to place this solution deep within the root of the tooth. . . 
I am curious as to whether you know whether the dentist placed a fiber (white) or metal (dark) post after root canal therapy. If a metal post was placed (unlikely), the color will show through the front part of the tooth. I wouldn't advise replacing the post as it would significantly weaken the tooth structure. I believe the darkness from the gums is showing through from the darkness on the root of the tooth. However, it is difficult to rule out whether the crown is invading the biological width of the gingiva (crown margin placed too far under the gums). If that is the case, a crown lengthening surgery followed by crown replacement could possibly fix the color. However, this will change the natural contours of the gums.
I want to give you some other less desirable options. Evaluate your smile line. If your lip covers the margin of the crown, then it will not be noticeable by the average person. If you have a high lip line, some dentists give botox injections in the upper lip to cover gummy smiles ( I have not done this, nor recommend this). The very last option would be to extract the tooth, followed by dental implant therapy/bridge therapy ) However, implants have their own set of problems.
I have taken course from Dr. Frank Spear, a leading cosmetic dentist in USA. He teaches dentists across the nation & works with very difficult aesthetic issues. He may offer a consultation! I hope this gives you more of an understanding regarding your case. 
It is impossible to give any diagnosis without doing a full clinical/radiographic comprehensive examination. Thus, my advice should be taken as opinion only.
-Dr. Sami Shahhal