reducing number of colors

[gautxori]

which should be the best way to reduce the number of colors for any image? : i tried both
- colors n
- posterize n - colors n,
and i come to a different conclusion depending on the image: photography -better with posterization - vs. color drawing -better without.

Is there any better way of reducing the number of colors for a map that's been photographed?

[xnview]

colors is often better. Do you have changed dithering option?