Changing color pict in grey excepted one color

[Ofnuts]

(08-16-2024, 01:20 PM)Gypsie38 Wrote: An example, 
a color picture in the street with cars. I want to change this picture in grey excepted keeping the color of one car
Thnaks

This is an effect known as "partial or selective desaturation". Typically you duplicate the picture, change everything to grey in the top layer (Colors > Desaturate > Desaturate) then use the Erase (or better, a layer mask) to erase the grey car at the top so that the color car from the bottom layer shows through.

An example here: https://www.gimp-forum.net/Thread-Partia...with-a-car

But you will find many tutorials on the technique, once you know what to look for.