The auction workaround works however there’s a safe way to do it. It’s time consuming.
Daily auction limit is 25 cars. List 25 cars you don’t want. Cheap from the autoshow or dupes from the playlist. When you get to the max you’ll get a message stating so. Then go to the cars with paints you want to delete. Here I only do easily replaceable cars to be safe. Send the car to auction. The game will tell you your at max and not auction the car but it will disconnect it from the paint file. Then go delete the paint file. It’s annoying but has always worked for me in fh4 and fh5. A beneficial side effect is your game save file is smaller and should take less to sync.