This is hilarious! Your coloring job is admirable, and the characters look great. I really think I could learn from your artwork on coloring effectively with watercolor paints.
However, there is one flaw that prevents this piece from being as successful as it could be. That is, the characters look so static. What I mean by that is, this is Mario Kart. Things are always in motion in racing games, but here, the go-karts don't really give off the impression that they're moving.
I suggest that you study pictures of race cars in motion. Play the game and see how the background blurs. Study the artwork of Mario Kart or some other racing game and see how the artists give the impression that the vehicle is in motion.
Other than that, you've created a beautiful piece. I love the bright colors and I truly do admire your artistic talents. You did a good job and I look forward to seeing your next piece.