Answer: Everyone heals at a different rate, so I wouldn't be too worried yet - if it appears that it IS healing (even if slowly) and there doesn't appear to be any severe redness or infection, then you're probably okay. Just give it a little more time and yes, once it is FULLY healed, the original color and brightness should come back. Looking dull is just part of the healing process. If you notice any dullness of color once it's fully healed, most artists offer a free touch-up within a certain amount of time from the original tattoo.
On a more personal note, I would suggest that maybe you see your doctor to see why your body is responding so slowly to heal. It could be an indication of a bigger problem, like Type 2 diabetes. Or it could simply mean that you're lacking some essential vitamins that keep your immune system healthy. Either way, this could be your body telling you that it needs something and this is a good opportunity to have it checked out.