In a lot of production drawings for anime (and I've also seen them in plots), particularly of characters as opposed to backgrounds, the detailed sketches/drawings have lines in different colors. Is there a purpose for this other than maybe to differentiate between lines? Are they used for color boundaries? Is there a production reason why the specific lines have to be different colors?
Just to be a bit more clear, I was wondering specifically with the colors used in these particular style line drawings, where the lines seemed to be color coded. For example, given this animatic, the first sketch has color used in shading and hue, but the middle picture shows the color coded line art, where the tears are red and the eyes are blue.