How to extract color from post process shader?

Hello !

I wanted to know if there is a way to extract some color from a desaturate post process shader ? I have a black and white game, and actually, I mange to extract object from the shader but I want to extract juste one color. I can’t do this in material, because as the game is going on, the color will appear gradually, one by one, and at the end, the game will be full colored… Any idea how can I manage this ?