Combine color levels with indexed conversion with single preview

[rich2005]

For a color indexed image, if you open the colormap dock Windows -> Dockable Dialogues -> Colormap you can edit the indexed colors directly Like this demo: https://i.imgur.com/LZikSHu.mp4

So if you have say 4 or 5 standard colours, you select and use from the colour selector.

Just a possibility.