The colour blue.

It turns out that Iranian forces made an earlier concerted attempt to seize a boarding party from the Royal Australian Navy. The Australians, though, to quote one military source, "were having none of it". The BBC has been told the Australians re-boarded the vessel they had just searched, aimed their machine guns at the approaching Iranians and warned them to back off, using what was said to be "highly colourful language".

This is why the Aussie armed forces aren't used on peace keeping missions. They shoot back. And use colourful language.