How to make a monochrome picture and bring back colours

All Mega1 channels have the same priority.

Take any Mega1 channel and increase it's input hue width to 360 degrees.
Then turn down it's output saturation.

The whole picture will go monochrome.

Now you can bring up any colours using the other Mega1 channels
Select the channel and increase it's saturation.

This leaves Mega2 fully available for other effects.