Take a screenshot of a typical scene in your game.
Open it in Photoshop and convert to grayscale.
Color correct it using the Image->Adjustments->Curves.
Save the .acv file file from the dialog using Save…
Open Pro Standard Assets->Image Based->color correction ramp.png in Photoshop
Now apply color correction to the ramp image: open Image->Adjustments->Curves again and load your saved .acv file
Select your camera in Unity and select Component->Image Effects->Grayscale to add the effect. Select your modified color ramp.
Hit Play to see the effect in action!
Color remapping works by remapping the original image luminance through the color ramp image (sized 256x1):
result color = pixel’s color in the ramp image at (OriginalLuminance + RampOffset) index. For example, to invert the colors in the image you only need to flip the original color ramp horizontally (so that it goes from white to black instead of from black to white):
A more complex version of color remapping that does arbitrary color correction can be achieved with ColorCorrection image effect.
This effect should run on all hardware that Unity supports.