The task is simple: Just guess the right RGB values for given colors.
As a start, there are just two possibilities for each color channel 0 or 255 - resulting in only 8 colors you have to know. Easy, isn't it?
After a while the difficulty rises and if you master level 4 (729 different colors) and reach level 5 with its 4913 colors you're quite good and probably professionally trained. A painter, designer, ... ?
But still, this is so far away from the 16777216 colors in level 9 - where we don't expect anyone to get anytime soon. Besides, the most mobile displays can't show these many colors anyway.
Why? Just try it and see how you get better and better...
NEW: Hexadecimal mode to make our fellow web designers happy!