My favorite app for working on C64 pictures!
On macOS 15.x you may experience that Multipaint won't start anymore. Even with the known tricks for unsigned apps...
There is an app for that called "Sentinel". Search at GitHub for "alienator88/Sentinel" and be your own Gatekeeper!
Remember, you will NEED OpenJDK 17 (or higher) + Download the Multipaint P4 version!