I've been trying on and off for years to find a way to electronically apply some sort of colour wash to my drawings. Something that would add colour without destroying the original shading. Too early to tell what the limitations are here, but maybe, just maybe...
For anyone interested this was done as follows. Scanned drawing contrast adjusted for best definition. New raster layer added on top with blend mode = multiply. Colours painted on this layer with soft-edged brush. Once fully painted, layer duplicated twice. Lowest layer (directly above drawing) blend mode set to colour. Next layer up blend mode set to burn. Top layer blend mode set to multiply. Transparency of various layers tweaked to give best effect. (Tests were done along the way when selecting colours to paint with to avoid undesirable results.)
Hey, why's Leela got two eyes? Okay, different Leela Excellent drawing and there's no doubt this coloring technique worked very well! Personally, after the contrast adjustment, I'd have just grabbed the black/dark color with a color select and copied it as a separate layer and colored beneath it but while your methods are much more complex, you've clearly shown the path you chose works well! Good job!