Is there a way to have a saturation ramp for color correction?

I'm messing around with color correction in Unity, using a custom color ramp to fake a 16 bit effect. One thing I'd like to do is decrease the saturation as the ramp goes from light to dark, so that colors will move toward grey as they get darker. However I can't figure out a good way of doing this. Does anyone have any advice for how I might get this effect?