The reason Portside is Red and Starboard Green is down to the steering and mechanism of the older oar-driven boats.
Steering mechanism denoting Portside and Starboard
Early boats used oars for steering rather than a centralized rudder. The steering oar was typically on the right-hand side of the vessel. This placement allowed the helmsman a better view of the right side but obstructed their view of the left, making the left side the more hazardous or “danger” side.
Colour association of Portside and Starboard
To denote this dangerous side, the left side was often marked with the colour red, which has universally been understood as a warning or danger colour.
Passing of boats to protect the steering oars.
When boats approached each other, they would pass with their left sides facing each other. Despite the reduced visibility, this practice protected the steering oar—the most critical steering component—from potential collisions.
Similarly, when docking or tying up, boats would secure their left side (port side) to protect the steering oar, which was on the right side. This practice ensured that the most crucial steering component was less likely to be damaged in port.
For an easy way to remember this check out our post on remembering Portside and Starboard.