I'm not sure what the idea was here. I hesitate to criticize something I don't understand.

A few things could have made this a nicer game IMHO: 1) Maybe add something to do with the rainbow (hit something, collect something?) 2) The rainbow is so dark that at first I thought it was black. Then I really squinted and I realized there are some colors in the black. Bright and colorful would have been much nicer. 3) An option to go full screen, or at least a larger screen size. 4) Some indication that the rainbow is actually created by us. There is some delay between when I hit the button and when the rainbow starts being drawn. At first, it felt to me that the game draws the rainbow on its own.

The game kind of didn't work. Instead of a rainbow it just created a black line, and the arrow showing where it would go was partially out of screen. But still, since it is your first Trijam, I'm impressed you got something up within just 4 hours. Keep going at it, you'll do better next time!