https://www.posetteforever.com/viewtopic.php?f=68&t=3030&p=37807#p37807
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Nik
Thursday, 13 December 2007, 07:21 PM

Using GIMP Indexed Mode.
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Disclaimer: There are probably three better ways to do this...

I reduced my total count to 16 because I was changing all of a 'simple' gray-scale image If you need to tweak a couple of shades of a more complicated palette, GIMP can index up to 256...
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Using GIMP Indexed mode.
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HELP section is 14.9.6.1 in current version 2.4.2, but F1 or right-click gets context-sensitive help.
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Open GIMP.
File, open, browse to piccy file. ( DO NOT experiment with original !!)

Image, Mode, Indexed. 
 ( New dialogue box opens)
 Convert to indexed colours.
 Select 'Generate Optimum palette'.
 Maximum number of colours ?
 Select manageable number, eg 16 )
 Convert.
 ( Dialogue box closes)

Dialogues, Colour Map (NOT Palettes !!)
 (New dialogue box opens with actual palette of piccy )
 (Drag box wider until palette items big enough to see and/or neat grid !!)
 Double-click on palette item.
   (New dialogue box opens with colour editor.)
   Drag sliders and/or enter eg RGB values to taste.
   Click Okay, colour changes are copied to piccy. (!!)
 
 Repeat with other palette items until happy.

File, Save As...
 New dialogue opens: Must convert to RGB before Export: Convert ?
 Click Yes.
 JPG dialogue may open, offering compression default: Set and click Yes.

Aaaand, there you go.