Hello! I’m not really sure what you mean by “soft grey colour”, I haven’t done any colour changes to the line-work in step 3, only in step 4. The shadows and blocking (which I think might be the grey effect you are looking at?) are done with really short strokes and occasional light outlining so I don’t go outside the lines, cause I’m shit (lazy) at cleanup.
Tumblr re-sizes the images so it’s hard to show something that is close up without making a bazillion posts, but I hope looking at it closer might help answer some of your questions: http://sta.sh/08ysw8xjg8s
This effect can be achieved in any medium :)