I wrote this script to allow the quick separation of Red Green & Blue channels from Matte passes. Download Here:
Just drop the shuffleRGB.py script into your .Nuke folder and add the code below to your menu.py file.
This action will also create a keyboard shortcut ‘alt + r’, just delete this bit if you don’t want it.
m = menubar.addMenu("&CG Scripts")
m.addCommand("&Shuffle RGB", shuffleRGB.shuffleRGB, 'alt+r' "")