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MacOS caches icons, which is a good thing for speed and such, but when tinkering with icons this can be a pain.
Instead of your "new" icon, your old icon remains showing no matter what you do, or you system starts showing generic icons, which is not wanted either.
The Icon Cache can be cleared with the following Terminal statement (it is one single line, requires your admin.sudo password and of course; use at your own risk, I just tested it with Mojave and it works):
sudo rm -rfv /Library/Caches/com.apple.iconservices.store; sudo find /private/var/folders/ \( -name com.apple.dock.iconcache -or -name com.apple.iconservices \) -exec rm -rfv {} \; ; sleep 3;sudo touch /Applications/* ; killall Dock; killall Finder
(source)