Hello Karl,
i know you asked for a recolouring tutorial in Gimp.
But allow me an advice:
i also started using Gimp, but i soon changed to Inkscape since it is quite more favorable for this kind of work.
With your allowance here is a Link for a youtube Video recolouring with inkscape:
https://www.youtube.com/watch?v=UM7t6n6BrC0
Gimp is very good, but it is pixel based whereas inkscape is vektor based which is quite better for recolouring cardboard models.
If you go on using gimp i would, beside that give Inkscape a try too, it's worth it!
Yours sincerely
The Oldenburger