GameMaker: Studio!
I was gonna start in Unity but I didn't want to go through the pixel perfect pain. Perhaps I will use the GameMaker version as a prototype and switch to Unity later still got 8 days.
No! Sorry. In computers a color consists of 3 components (Red, Green and Blue or Hue, Saturation and Value) combined together. In this jam, you can't use except 4 "computer" colors. So for example, you can use white, red, blue and green ONLY or you can use green with 4 different shades ONLY (which most GBJAM participators use).
Hope that helps!