Recreate traditional inking
If you look closely, many traditional ink drawings have a subtle bleed around the line when colour (especially watercolours) is applied over it. This leaching of the ink can be recreated by adding a Drop Shadow to the line art in the same colour that you’ve chosen to fill the line with. I set the Opacity to 20 per cent, the distance and Spread to 0 and the Size to 20 pixels.