color

hello everyone

I'm trying unsuccessfully to create a composition that has a video output in black and white + one color I choose. An example would be that all green objects in the scene appear green and the other black and white

for what little I understand. I would create a flow P / B and one where only certain color was visible and the remainder is alpha, similar to a cromakey. then join the two streams on the billboard. (Can not do this blending) the only way I know to do is mix with blendings modes

or another form where I make a color selection ..

I'm trying, if someone help, I will be grateful.