This is a collection of brushes that draws two lines at once. I use them for stuff like shoelaces, coat hangers, thin tails, stuff like that. :o)
Most of them has a white fill in the middle, which means you can layer strokes on top of eachother. To colour in this white space, you can either make a multiply layer above, or go to Edit —> Convert Brightness to Opacity, which will delete the white part.
Before you do this, you want to make sure you’ve merged the layer of the lineart with the layers of the double outline brush, otherwise it won’t work as well. Example:
Enjoy! :D
Brushes