There are two methods I use frequently for scanned lineart. The first is good for finished lineart that needs no colour changes or sketches that you need just the shape. The second is good for very sharp lineart that you need to colour differently.

Step 1: Duplicate your background layer. I do this to most images as the background layer is locked for some types of editing.

Step 2: Create a new layer underneath Background copy and fill it with grey.

Your layer stack should look like this:

I usually ctrl-E on the grey layer to collapse it with the background.

Step 3: Select the background copy (the layer with lineart on it) and hit the drop down box about the layer locks. Select multiply from the list.

What this does is make white transparent, and black opaque. Screen has the opposite effect.

Step 4: Create a new layer under the lineart and start colouring- have fun!

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