Digital Light & Color

Would anybody know how Monocolor actually works? What are the thresholds that determine what's colour, what's gray? I experimented on test image a bit, and it appears that if the chosen RBG channel has a higher value that the other two, it retains some saturation, but I'm still not sure if this is true.

Also, what would be a good preprocessing idea before Monocolor, to influence what's included in the 'color' part and what becomes pure gray?

Monocolor attempts to render the image as accurately as possible using just varying amounts of white and the selected color and preserving luminance. The math - taken from a technical journal - is a little complicated and there are no thresholds as such.