Changing colors of parts of an imagein GIMP

[hedden]

I know that there are many "how to's" about changing colors in GIMP, and I've been able to do this in a few easy cases, but I don't know how to do what I want to do now, which is to change some parts of an image to one color, and other parts to another color.
In particular, take a certification seal such as shown on this page:
https://www.atanet.org/certification/ct-designation-and-seal/
Say I have a certification for French. The idea would be to colorize the stamp so that it has the same colors as the French flag (blue, white, and red), arranged in vertical stripes as in the French flag. That is, the left third should be blue, the middle third should be white, and the right third should be red. Ideally, the white part should have a black border, in case the stamp should be placed on a white background. (The image background should be transparent.)
If the certification is for German, the stamp should be colorized so that it has the same colors are the German flag. Etc.
Can anyone explain the easiest way to do this?
Thank you in advance for any suggestions.
Tom

[DuckHook]

It's been ages since I've tried to do anything complicated in GIMP, so take this as the intellectual exercise it's meant to be. In other words, I have no useful examples or specific instructions for you, but..

Would it not be possible to do this using transparent layers and masks? One mask per layer so that your colouring applies only to the unmasked portion of the image.

The other meta-suggestion I have for you is to post in the GIMP forums. The official forums are linked to here: https://www.gimp.org/discuss.html#forum
However, they deal more with the app itself (i.e. feature requests, bugs, workarounds etc) rather than usage. For unofficial user supported forums:
https://www.gimp-forum.net/
or
https://www.gimpusers.com/forums

You will likely find more of the actual graphical userland experience on those sites rather than on a technical help site like ours, which focuses on Ubuntu as a distro/OS and not so much on specific apps that happen to be in the repos.