Using Clipping Masks which is a very handy tool that I use almost of everything; toning, shading, highlighting and colored lineart.

1. ( of course) I always line on a new blank layer

2. Color lineart using clipping masks using one of the 2 following methods;

(a) Slow:

-Add New layer

-Right click new Layer > pick “Create clipping mask”

- Color

What clipping masks does, is that is takes whatever is there on the layer it’s clipped to and use it as base for whatever is drawn on it. It adds the colors only on the lines that are there on the lineart layer.

You’ll notice that if you remove the clipping mask (by right-clicking the layer>Release Clipping mask) the colors are there the way you painted them on the layer. The clipping mask made the colors only use the lines in the lineart layer as base.

(b) Fast (shortcuts way)

-Adding new layer by clicking ctrl+shift+n (Windows) / Cmd+shift+n (Mac)

Check the box “Use Previous….”

You’ll notice that you can make multiple clipping mask for same layer (as many as you want)

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At last; there is a fastest way for coloring lineart that I never used myself because I kinda like to keep the black lineart as is without duplicating it.

You can do it by locking the transparent pixels on the lineart layer clicking the first small button at the top of the layer window

It’ll add a lock to the layer and you can paint over the same “lineart” layer with the colors you want  without needing any clipping masks.