Thanks for the feedback !
That is exactly as Katelina77 said.
We have 2 possiblities to change colors :
The first one is the plaftorms with particles. When you stomp on them, you take the color of the platform.
Then, you also have some backgrounds that can be mixed with you. In that case, you add your color to the color of the background. For example, if you are green, and the background is red, you mix both colors and become yellow. You keep this color while the background is still behind you.