Hey all, I'm an artist that uses Krita, and a novice programmer. I had an idea for a blending mode and wanted to know if such a mode already exists, and if not, is there documentation for how to create blending modes?

The idea is a variation on the color mode. Using color for colorizing grayscale art works fairly well, but the color often ends up looking flat and digital because the hue is the same regardless of the underlying value, and that is not how reality works. I think a fairly easy fix would be to have a blending mode where it treats middle grey as exactly the hue chosen by the user, lighter than middle grey gets slightly warmer over time, and darker gets slightly cooler over time. Does this exist? Is this clear? How difficult would this be to code?

As a guy who like to play with color and light I can say this color mode won't help much, there is too many cases when it won't give right result.
Coloring grayscale is very tricky, I'd suggest to use another way: fill the colors first and use multiply mode to add shadows, it makes nice realistically looking picture plus it's super fast and efficient. I prefer to use it only as first step though.

But if you want to try anyway look at this: https://phabricator.kde.org/D5903
It adds new mode, so you'll know where relevant code is. Not sure how hard it to be.

I'm not sure I understand. I am suggesting the Color mode isn't as useful as another similar mode might be in coloring grayscale. Here's a painting I'm working on. I've used the red to illustrate what I mean. It'd be cool to have a mode where a lighter value underneath changed the hue a slightly warmer color, and cooler for darker values. Is that what Hard Overlay does (thanks for the link)? In what version of Krita is it available? I'm still using 3.1.2

1) I doubt blending mode you described can help much
2) imo: instead of coloring grayscale draw\paint with flat colors and use multiply mode to add shading, it's easier, faster and gives pretty good result
3) if you want to try to create blending mode, here is the link to patch which add new blending mode, just to show you where code the for modes is
4) hard overlay doesn't do what you described, it's mix of multiply and color dodge modes

You can try to use a Filter Mask with a Gradient Map Filter on it, applied on the Grayscale Layer. (This is heavy)

Artur: Filter mask could give a similar result but you'd have to match all the values manually in the gradient I think. But fiddling with it has made me more confident that a mode such as I described could be useful.

Radian: As for the multiply process, thanks, but my point wasn't really that I need help coloring grayscale. I paint mostly full color with no blending modes, but a lot of people start with grayscale, and it occurred to me this could be useful for that popular process.

I'll try and look into it more on my own. Thanks!!