It doesn't have to be that complicated. All you need to do is randomly pick one of the 3 values to be a low range. You can do it with one event :)
Why not set color to:
choose(rgbEx(random(0,25),random(75,100),random(75,100)),rgbEx(random(75,100),random(0,25),random(75,100)),rgbEx(random(75,100),random(75,100),random(0,25)))
I've tested this and I think it works pretty well :)