11-04-2021, 06:47 PM
There is always a way, although not too sure that Gimp will retain the geodata 
Try
1. Layer -> Duplicte the layer
2. Colour pick that yellowish color
3. Color select the white background
4. Select -> Grow 1 pixel
5. Fill the selection with White (drag in from the BG swatch)
6 Select -> Invert (the selection)
7 Fill with the FG 'yellowish' colour (drag in from the FG swatch)
8. Select -> None to turn the selection off
9. Turn the visibility of the top layer off and make the bottomlayer active
10. Colors -> Threshold to give a BW layer
11. Back to the top layer and make it visible
12 Set the top layer mode to 'Darken Only'
13. Layer -> Merge Down to merge the layers.
Lots of steps but I can get in 1 minute demo https://i.imgur.com/A2N8D5e.mp4
There will be other ways.
Try
1. Layer -> Duplicte the layer
2. Colour pick that yellowish color
3. Color select the white background
4. Select -> Grow 1 pixel
5. Fill the selection with White (drag in from the BG swatch)
6 Select -> Invert (the selection)
7 Fill with the FG 'yellowish' colour (drag in from the FG swatch)
8. Select -> None to turn the selection off
9. Turn the visibility of the top layer off and make the bottomlayer active
10. Colors -> Threshold to give a BW layer
11. Back to the top layer and make it visible
12 Set the top layer mode to 'Darken Only'
13. Layer -> Merge Down to merge the layers.
Lots of steps but I can get in 1 minute demo https://i.imgur.com/A2N8D5e.mp4
There will be other ways.