I have been using this image (painted in Krita) to test the fill tool in all my software. They are: ArtRage, SketchBook Pro, Photoshop, Clip Studio, Painter and Krita. The only software of these that can not fill on a layer, without selecting or masking, under the line art, is Painter. That is why I do not use it anymore...

All the other can do it with varying results. ArtRage and SketchBook Pro are not really useful for real good fill, they leave some pixels around the edges unaffected as a visible ugly fringe of pixels.

Krita, Clip Studio and Photoshop gives the best results, really pro, making the fill cover up the overlaying line art edges.

In Clip Studio there is a "Close Gaps" function. Using that makes it possible to fill the girls face without spreading the paint all over the layer. You can see the gap between the chin and the hair that Clip Studio "closes" and makes the fill without spreading. Nice! Shoud be in Krita too... or is there?

Krita makes filling easy..

hey. I don't believe there is a fill gap setting currently with Krita. I remember it being discussed before.

Clip Studio Paint does have some really nice fill options. One of my favorite ones is the ability to drag and fill. In other words, you drag over all of the areas in one swipe and it fills all of the different areas in. Great for things like clothing that might have a lot of different areas.