Hey, man - thanks for this. After extensive searches on the net, yours is the only tutorial I've found. I had to use a less than perfect altenative up til now. I'd left off doing old school line art 'coz I couldn't edit the scanned line art layer how I wanted too. Nice one.
It selects all the white and greys, then when you hit delete it clears all the white of your layer and makes the greys go transparent so when you add colour underneath your sketch layer the greys shade your colour instead of having white in the grey. Are you holding all 3 keys at the same time like i'm asking?)