NOT a brush that overlays
It's basically simulating the effect you get when you put a colored layer set to overlay on top of your linework, as the main color being the linework and the sub color as the color.
Blending mode is set to linear burn because I mostly use this with black but if you're planing on using it with lighter colors I reccomend changing it to multiply or something else.

I have no idea who else would use this