In Paint tool sai its called “Clipping Layer” xD So that might be why you couldn’t find anything. But yes!! It makes coloring 500x easier (at least for me since i color like a 3 year old).
Step (1): Select everything OUTSIDE of the lineart then Selection>Invert.
Step (2): Then Fill the selection in with a color (preferably dark so you can see whats not colored)
Step (3): Set that color (BASE) below the Lineart layer then create a folder above the BASE, then clip it. Clip the entire folder to the BASE layer.
How it should look:
Step (4): Just create a new layer for the colors you want INSIDE the folder. If it is outside the folder its not going to “Clip” to the base and the colors will leak out.
Step (5): Shade and finish~
But yep that’s how I do it for everything I color unless its 1 layer stuff. You can get really fancy with all this Masking/Clipping, like I do with my effects all the time. Sorry if this is messy I left my tablet at home aha xP.
Hope I helped QwQ/