reduce the available colours (palette) in GIMP to only two

Question

I drew a picture of my (future) living room and manipulated the photo's brightness and contrast until I arrived at the attached sketch.

I managed to replace stray colours in the palette with Colors --> Map --> Colors Exchange (not without carefully noting the HTML colour notation, as the colour picker was of no use here). My aim was to reduce the picture's colours to only two, i.e. black and white. Then, I wanted to increase the colour palette back to a larger number to allow for the Bucket Fill Tool to colour areas (i.e. the walls). I want to show to my wife what different wall colours might look like. I found this (what I imagined) simple task of increasing the number of colours in the palette unsurmountably difficult and I didn't manage to colour areas in the picture.

Any simple ways for colouring areas in the respective picture (that does not require studying tomes of GIMP guides)?

Answer 1

1. To make you picture black and white, it is much better and faster to use Color > Desaturate > Desaturate, and possibly add some contrast to have deep white and back (but don't overdo it to keep smooth lines)
2. Duplicate the layer
3. In the top layer: replace white by transparency, keeping only the lines:
   • Layer > Transparency > Add alpha channel (if this is disabled the alpha channel is already there)
   • Color > Color to alpha and remove the white
4. Add a layer: Layer > New layer, let's call it "Colors".
5. Move it under the "Lines" layer: Layer > Stack > Lower layer
6. Start the bucket-fill tool.
   • Click Fill by line art detection
   • Set Source to All visible layers
   • Check Fill transparent areas
7. Set the foreground color to the required color and click in an area to color. You may have to click several times.

You can also work with the "Colors" layer on top but things won't look right until you move it under the line art layer:

Advice: The "Line-art detection" is quite CPU intensive, on your initial image it brought my rather capable PC to its knees. Making you image smaller (1200x900) (use Image > Scale image) makes it much more responsive.