I remember this thing, but I can't find it, I feel stupid or something.

Ever see linework, where the shadows are outlined by hand, perhaps with a different color? It feels like an extremely advanced technique, but i can't find it, I might not know the right terminology.

I think this is how it works.

blue linework for the draft.
ink directly on it, make sure all linework is enclosed
then red lines at the edge of where the shadows will go, making sure to connect both ends of each line to the black linework
scan
filter out the blue
duplicate linework layer, lock, hide.
filter out the red
levels to remove greys.
paint bucket in all the colors
magic wand all the areas between red and black, paintbucket appropriate colors
hide the layer with the red linework.

completed cell-shaded drawing
no erasing pencils needed
don't need to scan twice
no time wasted handcoloring