Use a separate layer for colouring if you're worried about messing up your line drawing. I find that helpful when I'm learning to use the tools in an unfamiliar CG program.
I think that this line drawing is more artistic and pleasing than a copy of the image already. I would just make it black the same way and finsih it like this.
drawn in 23 min
Link to pic!
http://www.cswu.cz/epizoda-iii/images/anakin/122.jpg
Tbh im not good either with grayscale.
This is fabulous, dude.