Selection and Application by Color

Is there are way that I can only effect the roughness of the green (or any targeted color) segment of my texture with out applying a separate alpha map. Is it possible to generate a mask based on a targeted colour using nodes?

Thank you!

This setup will bring down the roughness on the green.
Use the eyedropper in the color of the add node to get the right color.

It can be switched by playing with the sliders in the color ramp in this setup.

Actually two of those nodes was not needed.
Here is a simpler setup. Again play with the sliders.