Were only alowed 4 colors, but can we do different shades of those 4 colors?

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!