[split] Fake colorization

[Ofnuts]

You can try this:

1. Make a duplicate of the image
   
2. Run Color > Desaturate (Luminosity) on the bottom one
   
3. Insert a gray (#B8BB8) layer under the top image
   
4. Set the top layer to HSL Color mode: you will obtain an image of flat color areas (the group thumbnail, below)
   
5. Layer > New from visible to obtain a layer from this
   
6. Hide the gray layer and the image copy (you should only have the new and the desaturated layers)
   
7. Set the new layer to HSL Color mode, you should obtain your initial image (more or less)
   
8. Apply Colors > Posterize to the top layer to reduce the colors
   

In the screenshot below, I used a group that shows the result of the color extraction but it is not necessary.

Original:

   

Result:

   

(yes, this could require some preprocessing of the sky to make it more uniform, and this would have let me add a few more colors in the posterization).