In Mint 21 has old buggy GIMP whose color picker does not work. The solution is to remove this version and install ppa-version
apt purge gimp
sudo add-apt-repository ppa:ubuntuhandbook1/gimp
sudo apt update
sudo apt install gimp libmypaint-1.5-1 libmypaint-common gegl
Without libmypaint-1.5-1 and gegl packages you get the next error, so they are crucial for GIMP.
gimp: error while loading shared libraries: libmypaint-1.5.so.1: cannot open shared object file: No such file or directory