I am very new to Krita and digital painting in general. In my current file, I have the default layer that's created with all new files, a paint layer called "backdrop" that I completely painted, a lineart layer that's an inked version of my sketch, and another paint layer between the "backdrop" and lineart layers that's called "flat color". I've painted on "flat color" and the color shows through the lineart, which I want. However, I don't know how to get the paint to not cross outside the boundaries of my lineart. I've read the articles about masks and layers, as well as the forum posts here and here, but it's all gone over my head. Is what I want to accomplish possible with the way I have my layers set up, or do I need to be doing something different? If my layers are fine, how do I go about restricting the color to the boundaries of my lineart?

You flat colors will need to be filled manually through one of the many means: https://docs.krita.org/Flat_Coloring

After that, you can lock the transparency on a layer by ticking the little checkerboard icon. That way, you will be able to make detailed adjustments like shading or drawing patterns on that layer.

@TheraHedwig I think I understand what the article is saying. Is there anything such that, when I attempt to color outside the boundaries of the lineart, the color just doesn't appear? Or am I responsible for keeping the coloring neat? I thought some art programs had a feature that could make lineart act like a "wall" even if the colors were on another layer, but I could just be misunderstanding how these programs work.

You can try to use the magic wand to select inside the lineart, but i doubt it will be effective.
In my opinion, the easyest way wold be this: https://www.youtube.com/watch?v=YigbVY9s6gU

xscizor wrote:@TheraHedwig I think I understand what the article is saying. Is there anything such that, when I attempt to color outside the boundaries of the lineart, the color just doesn't appear? Or am I responsible for keeping the coloring neat? I thought some art programs had a feature that could make lineart act like a "wall" even if the colors were on another layer, but I could just be misunderstanding how these programs work.

Yes, clip paint can do this, but Krita is not clip paint and the other way around(And the clip paint method doesn't work on colored lineart, as far as I know).

The standard practice is to carefully fill in yourself, and then lock the alpha so you can just doodle all over.

xscizor wrote:... Or am I responsible for keeping the coloring neat?...

If you line art is strong (color wise, e.g. pure black) and completely closed (for the fill not to "leak") just use the fill tool, let me simplify the steps I gave in the link you posted above with a 4 layer example:

Let's say you have four layers (bottom to top) whitebase-sketch-colorfill-lineart.
-Select you colorfill layer and hide sketch layer
-Select the fill tool and make sure is not limited to current layer (default)
-Fill as desired

Explanation: Here the fill tool will take in account your top layer lineart as far as there are no interfering layers below, that's why we hide the sketch layer. the bottom whitebase layer works as a color sample, all white will be filled within the confinements of the lineart layer.

As a side note: I rarely fill color like this, having a pen makes painting really easy, serves as practice and at times is even therapeutic , now if you have only a mouse, I can understand the requirement of filling color the way you want. My personal way of coloring is (usually) negative, this is I color without paying attention to the lines, then erease/trim the uneeded color, just to mention one of a miriad of coloring techniques. : )

It may not be perfect, but try selecting, using the freeform select tool, the area in which you would like to paint. While an area is selected, only the inside may be edited.