Nice technique. How about using the colour ramp node to get three masks out of a greyscale image? Draw one mask in white, another in 66% grey and the third in 33% grey. Set up your colour ramp with three blacks and one white colour - white at the top would mask the white, white in slot two would do the 66% grey and white in slot three would do 33% grey. Not as powerful as your colour technique, nor would it allow as many masks, but it might be handy