IF I were to implement this in xLights (big IF), it would likely be a modification the dimming curve dialog for the panel where there is a separate dimming curve for each color. Basically, if the "green" dimming curve is 0% the entire way, have an optional checkbox to have it zero out the others if there is a green component or similar.
Really, when I wrote the dimming curves, I had thought about trying to allow some sort of pluggable functions or similar in there that the user could write something. However, the dimming curves are on a critical performance path so setting up a scripting engine and running a script or whatever would have killed the performance so I never pursued it.