Definitely one of the best games I have played so far.
I really like the art stlye. Especially the background. The idea is really nice and the levels are designed very well. There is just a small problem with the order of the levels. As already mentioned by EnT_JLR, the jump from level 2 to 3 is madness. The order for the levels should have been only SingleDan and introducing all the colors and then changing to DoubleDan but also decreasing the amount of colors so that the player has time to get used to keeping two Dans alive. You could also think about the rate you are introducing new mechanics. But yeah, that's way too much for a game jam and I didn't rate your game any worse because of that. Just something to keep in mind for the future.
Oh and btw. why is the fourth button using 3 colors? Orange, brown and cyan? Or did I mess something up when clicking around in the options?
One more tip. Maybe keep the button-color-information on screen all the time. Not that big as at the start, so that the player can make a quick peek to see the correct button. Especially at the start or introduction levels.
I hope my feedback helps you.