Shading layers would go above the layers you want them to affect. If they are in a group, they will only affect the layers under them in that group. They come in 2 types: Shadow and Highlight layers. These are very similar to the shading pencil, in that they just take the color they’re on top of and move it to the left or right in the currently-indexed palette–the color you draw with on those layers is completely irrelevant.
When you have a shading layer above your current layer, you can press S or H to draw on the closest shadow or highlight layer above your current layer. Pressing either of those keys again takes you back to drawing on your current layer. However, even though it’s not being drawn on, the layer you’re drawing on isn’t deselected–if you’ll notice, the orb layer is never deselected in the provided example gif!
tl;dr shading brush as a layer
This would greatly simplify my workflow, and I have a really cool idea that would pretty much only be viable to produce if this were a thing, so plz add it :3
I’ll answer any questions and edit them into the main post!