Select Colours - Poster Colours

Check Before/After image

the default is 3 colours, you can use the slider bar up to 256 colours to see the before/after diffence

In general a German CoA for instance can be reduced to below 10 colours

Maps with highlighted areas + CoA can often be reduced to below 15 colours, without visible difference

French CoA often have light effect, and therefore need more colours - sometimes 100

So it very much depends on the image, but the before/after will tell you how much you can reduce

For the colour reduction to have an effect, you need to export the image to GIMP directory