krita already has an operator that breaks apart a layer into different
layers by color. It also still has gmic, which allows single frame
illustrators to use a layer for each color (a feature I suggested to the
author). For now I say - stick to Deevad's mockup proposal for a single
layer coloring (the way it is with the original lazybrush implementation on
tvpaint) - as it is better suited for animation.

I really like Deevad's mockup. It seems to fit well with krita's overall design and the functionality that we require for animation. This colorize mask would of course need to be exposed to the timeline in the same way that the standard lineart layer is exposed.
The current gmic implementation can also do "replace color". Replacing a color globally with another color is usually an advantage of vector based animation tools, but if we get that in krita as well, it would be amazing!

Here is how lazybrush CTG layers work in TVP11 for reference:
https://www.youtube.com/watch?v=FDjQNvbgzmw