Posterize or make contour lines by defined colours

[pygmalion]

I have a picture as a result of a calculation program:



There is also a legend for values:



Now, I would like to make a picture with seven colours (posterize) OR find six isocolor lines.  The six threshold colours can be extracted from the legend with equidistance.

Is something like that even possible in GIMP?  When I make posterize in GIMP it does the job but I don't like the colours that were automatically selected as threshold colours