This was extremely cute and I like the message it conveyed. The gameplay was very simplistic and in another game where the mechanics didn't complement the other elements, I would call it pretentious. Would being the key phrase here.
For me, I feel that the gameplay was somewhat metaphoric. Your walk through the game was more like a walk through time, the character meets adversity at first, but slowly gains friendship until the end. Going left is a central theme of the game because it's a breaking of super traditional side scroller law to go right, thus being different. Or maybe I'm reading a bit too much into the gameplay.
However, while I like the story and I liked the gameplay, there were some things that perplexed me. Namely, the black and white/us and them mentality of the narrative and its utilization in the game. The "boring" "stuffy" people are populated in the gray world while all the "cool" people get the psychedelic colors and brightness. That would have been fine if not for the main thing that vexed me. The ability to go back.
I noticed that there was a dude on the way to the colorful land saying "don't look back". Once I got to the trampoline, I noticed a hole I could go out of. I tried going back a little bit to see what would happen, all that was there was repeating dialogue.
Even though I like the simplicity, I still wish the story could be expanded on. If you want my opinion, you can have a complex story and still be simple. The main way to do that is to add a moral gray area. To use a cliché, the world is not always black and white (or in this case, bland and colorful). Why do the gray people hate the colorful people? Why is one side colorful and the other not? Who is the 1 guy and why does he help you in your plight? Can't we all just get along?
This may not be what you were going for story wise, but I honestly see a lot of potential for this and I would hate to see it go to waste on account of the fact that it was "just made for a game jam".