I posted this script upon a request coming from a Gimp user.
The assumptions are:
1 - the user had an original black&white drawing
2 - someone did "overpaint" the drawing with light watercolours
3 - the user likes to restore the original, as much as possible

The available Gimp functions (desaturate, grayscale) do not get rid of the gray nuances generated by the converted colours.
This filter is written on the basis of the colours theory (in painting) that any added colour does not really "add" but "remove" instead the complementary colour; for instance, if you have a white area and you overpaint it with pure Yellow, in practice you remove the blue component from White (yellow = red+green).
Ofnuts told me there is a manual quick solution, but -as you know- I'm quite lazy and prefer having a script performing what I want.

Here an example:
1-original b&w drawing

2-overpainted with watercolours

3-outcome after running the script


output is not perfect (some black pixel was removed during the overpainting), but IMO acceptable

(note that this script makes use of the "tile" mechanism in python, as found on an example of Maldonado; you may use this method any time you have to deal with each byte in an image)

the script is already posted (thanks, Rod !) under GIMP Scripts and Plug-ins, here)

Thank you very much for Your plugins.
Small cosmetic attention to the names in the menu (in my language - it merges).

thanks, Wallace: its use is a bit "specific" to some situations
thanks, MareroQ: I will take note of your suggestions for my next scripts definitions; for those already posted maybe Rod (who took the burden of posting them) can apply the modification
(he is totally "authorized" to do it....)

Otherwise, to do the same thing manually:

- Color>Components>Decompose (RGB)
- Put the top two layers in "lighten only" mode
- Merge down

All of 10 seconds, faster than hunting down the script in the menus

an advantage of the plug-in is to provide to the script-writers a skeleton to quickly perform pixels manipulation (obviously it was available already in the original Maldonado's script, but this doesn't seem to be "popular" here around)