####Remove custom icon from folder your_folder:
cd your_folder; rm Icon$'\r'
After executing this command, the operating system should immediately restore the default folder icon.
####Example usage:
cd ~/Pictures; rm Icon$'\r'
######tested with zsh 5.0.5 (x86_64-apple-darwin14.0) on Apple MacBook Pro (Late 2011) running OS X 10.10.3