That answer is absolutely incorrect.
You can set up a layer to desaturate any layers below it in the Blend Asset.
This is a basic setup for a Blend Asset that will use the Red channel of the top layer’s base color (you don’t have to use the Base Color, you can use any attribute of the top layer’s material you want in the Blend Asset. I just used base color because I threw this together in a couple minutes) to desaturate the base color of the incoming layer stack:
And this is the Material Layer that I’m using for the top layer:
(the saturate node is to cheaply clamp all the vector’s components between 0 and 1)
Here it is in action:
This is a super simple setup that just does saturation, but you can easily extend it to make essentially an ‘Adjustment Layer’ that you can drop anywhere in your material layer stack to tweak values of the lower layers.
[edit]Re-reading your original post I see you’re trying to increase the saturation, not desaturate. That graph is a little more complicated because you have to do RGBtoHSV conversion and back. Here’s what that looks like in the Blend Asset:
