Grayscale Green and Blue but not Red

Im trying to make a game looking like Sin City style, where all tones are in grayscale except the red tone.

Im using the Grayscale from Unity 5.2 in the camera and trying to edit it, but no success =/
Anyone have any idea?

ps: I’m totally shader noob here.

Thanks in advance.

I did the trick. Not the best way but i made it.

I made a a setup with two cameras.

The first with a Greyscale Shader attached.
Setup:
Depth = 0;
Culling Mask = Uncheck RedLayer;
Clear Flags = Depth Only;

The Second camera with no Shaders;
Setup:
Depth = 1
Culling Mask = RedLayer
Clear Flags = Nothing;

All i have to do now is put my red Objetcs in the RedLayer. =)

Not the perfect way, but it Works.