Changing thick lines to thin

[sallyanne]

You could try the magic wand on the black and delete it. It will not delete it all as the 'greys' will stay behind, but that will give you thinner lines.
For the white just add another white background to the back to fill in, which you could do first
I'm not sure how you are going to do the colour though. Maybe as you did before, or if you want plain red, just colour pick the red and fill in the spaces that should be that colour.