Posterize or make contour lines by defined colours

[Ofnuts]

(06-19-2020, 03:44 PM)pygmalion Wrote:

(06-19-2020, 11:56 AM)denzjos Wrote: Maybe this is a solution :
GMicQt / Artistic / Cutout 
Number of levels 7
Edge Simplicity 3
Edge Fidelity 2

Unfortunately, this tool takes wrong threshold colors.  I think that threshold colors must be picked manually.  But how?

Create a palette with the color you want, and use Image>Mode>Indexed and specify your palette as a custom palette. If necessary, copy the graphic to its own image and color-index it, and copy-paste the result in the initial image.