@anthracite oh that color outlining is excellent! Is that "normal-mapped" or some variation thereof?
Nope, it's just me telling Illustrator to draw three different blurred outlines around each shape. This is wholly 2d stuff!
@anthracite Cool cool! I was actually thinking in 2d; that's why I had quotes around the normal. You could at least in principle take the plane vector that's normal to the line locally and use that to index into a color.