Here’s how starboard became green
1834
Ships of the City of Dublin Steamship Company were equipped with white masthead, green starboard lights and red port navigation lights.
1836
The P&O Company of Southampton had a different arrangement; green for port, green and red for starboard.
1847
The British Admiralty ordained that starboard was to be green and port red.
1853
The Prussian Ministry of Trade prescribed the British lighting rules for its steamers.
1858
France, Austria, Hungary and the North German seaboard countries also signed up.
1889
27 other seafaring nations followed suit and adopted international maritime regulations ( known today as the International Collision Regulations ColRegs )
This colour code also applies to lighthouses. Lighthouses on the starboard side of harbours (when coming in from sea) are always green. Except for the Americas, Japan, South Korea and the Philippines which are opposite.