So I am working in Affinity Photo, and I have this pen sketch of a clown that i really dig. I would like to recolor/paint ONLY the lines in the work. Not the background. I would also like to vary the coloring (i.e. hair green, nose and lips red, teeth yellow, etc.). As you can see the lines are rather intricate so it's very difficult to "select" them and when "filling" the lines it loses the sketch quality. I've tried locking the layer and using a transparent background. But when I paint them, it doesn't isolate the coloring to only the lines in either method.
How would I go about recoloring/painting just the specific lines in the sketch various colors and keep the sketch effect?