You only touch things that have a color in common with you. For example, if you are yellow: yellow is made of green and red, so you touch all colors except for blue. If you are red, you touch red, yellow, purple and white, and so on. This is important in the boss fight too, so if you can't get through the level, the boss won't be much fun either...
If you just want an old fashioned boss battle, I recommend playing my other game Frost Heart instead :)