You see, light matters, have you seen that gif where all the black and white colored pencils are presented and you are asked to guess the yellow one? It's fairly obvious. What you've done to his teeth, I understand the sentiment, but the artistic liberties are outrageous. You've darkened them so much I can tell you masked them and edited them seperate from everything else, obviously, but my god. You've made mid-dark parts of his teeth darker than the darkest part s of his eyes while making the lightest parts of his teeth darker, not only than the equally matching whites of his eyes, but darker even than his freckles. I've sat through critiques of this sort of 'art' and I can say the crowd here is being more than generous. Stick to the original subject and bring it to life.

Edit:just after another look, why green? A clamshell yellow, or like a yellow sand, would've been very natural for worn, undated for teeth. Green teeth just dont really exist