Why is port red and starboard green?

The convention predates the modern rules and was fixed internationally in collision regulations. What matters is that it is universal, so the visible colour always tells you which side of a vessel you are looking at.

What does seeing both red and green lights on a ship mean?

You are looking at it from nearly dead ahead — it is heading towards you. Under the head-on rule both power-driven vessels alter course to starboard.

Are buoy colours part of the COLREGs?

No. Buoyage is set by IALA, which uses red and green lateral marks in opposite senses in its two regions. Only navigation lights and shapes on vessels are in the COLREGs.

Why do ships show a red light on the left and green on the right?

Short answer

Under the international collision regulations (COLREGs), a vessel shows a red sidelight on its port (left) side and a green one on its starboard (right) side, each covering 112.5° from dead ahead to just behind the beam, plus white masthead and stern lights. The arcs are arranged so that the colours you can see tell you which way another vessel is heading — and, combined with the crossing rules, which of you must give way.

At a glance

Rules: COLREGs 1972 (adopted 20 Oct 1972, in force 15 July 1977)
Sidelights: Red port, green starboard, each over 112.5°
White lights: Masthead 225° ahead; sternlight 135° astern
Colour limits: Annex I, by CIE chromaticity area

The standard

COLREGs 1972: Convention on the International Regulations for Preventing Collisions at Sea

Issued by: International Maritime Organization
Edition: 1972, in force 15 July 1977, as amended (1977-07-15)
Status: Current
Applies to: International waters and national waters of contracting states

The rules of the road at sea, including which lights a vessel shows, their colours, arcs of visibility and the chromaticity of navigation lights.

Colour meanings in COLREGs 1972
Colour	Meaning	Paired with
Red	Port sidelight, visible over 112.5° from dead ahead to just abaft the beam	—
Green	Starboard sidelight, over the matching 112.5° arc	—
White	Masthead light (225° forward arc) and sternlight (135° aft arc)	—
Yellow	Towing light	—

Status checked 29 September 2026. The convention text is published by IMO; the US Coast Guard publishes a free amalgamated version. Summary only, not navigational advice.

Summarised in Colourwise’s own words; not the text of any standard and not compliance advice. Always work from the current edition obtained from the issuing body.

Arcs, not just colours

What makes the system work is the geometry. The masthead light is white and shines forward over 225°, from dead ahead to 22.5° behind the beam on each side. Each sidelight covers 112.5° on its own side, over the same forward sector. The sternlight is white and covers the remaining 135° astern. Together they cover the whole horizon, with only one colour combination visible from any bearing. Seeing a white light above a green one means a vessel is showing you its starboard side; white above both red and green means it is heading straight at you; a single white light low down usually means you are looking at its stern.

Colour tells you who gives way

The crossing rule turns the colours into a decision. When two power-driven vessels cross, the one that has the other on its own starboard side must keep out of the way. If you see another vessel's red light, you are on its port side — it has you on its starboard side — so you are normally the stand-on vessel and it must give way; if you see its green light, the obligation is usually yours. Sailors learn this with rhymes about red and green, but the logic is the arc geometry. Other colours add information: yellow towing lights, all-round red and white combinations for vessels restricted in their ability to manoeuvre, and so on, each defined in the rules.

Port is left and red; starboard is right and green. A common memory aid is that 'port' and 'left' both have four letters, and port wine is red.

Colours by chromaticity, not by name

The COLREGs' technical annex sets the colours of navigation lights as areas of the CIE chromaticity diagram, bounded by corner coordinates, and sets minimum luminous intensities that fix how far each light must be visible. That precision matters because a navigator often sees a single point of light, far away, over water, with nothing beside it to compare. A lamp that drifts towards orange could be taken for a yellow towing light; a green that drifts towards white could be taken for a masthead light. LED lanterns must still fall within those areas, which is why navigation lights are type-approved rather than chosen from a catalogue.

Buoys use colour differently

Buoys and beacons are a separate system maintained by IALA, not part of the COLREGs, and there colour means the side of a channel rather than the side of a ship. The world is split into two regions that use red and green lateral marks in opposite senses when entering harbour from seaward, which catches out sailors who move between Europe and the Americas. The lesson is that a colour on the water carries meaning only within its rulebook. This page explains the logic of navigation lights and is not navigational advice.

The data

Basic navigation lights of a power-driven vessel under way
Light	Colour	Arc of visibility	Seen from
Masthead light	White	225°	Ahead to 22.5° abaft the beam on either side
Starboard sidelight	Green	112.5°	Ahead to 22.5° abaft the beam, starboard side
Port sidelight	Red	112.5°	Ahead to 22.5° abaft the beam, port side
Sternlight	White	135°	67.5° either side of dead astern

Source: Amalgamated International and US Inland Navigation Rules. From Rule 21 of the COLREGs as published by the US Coast Guard. Smaller vessels may combine lights; other vessel types show additional lights.

How to check it yourself

Note which colours you can see and their vertical arrangement.
Red visible: the other vessel is showing you its port side. Green visible: its starboard side. Both: it is heading towards you.
For the complete light combinations, read the rules themselves — the US Coast Guard publishes a free amalgamated version.

What the evidence says

Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.

FactStrong evidence
The COLREGs convention was adopted on 20 October 1972 and entered into force on 15 July 1977, replacing the 1960 collision regulations.
Source: Convention on the International Regulations for Preventing Collisions at Sea, 1972 (COLREGs)
StandardStrong evidence
Rule 21 defines sidelights as green on the starboard side and red on the port side, each showing over 112.5° from right ahead to 22.5° abaft the beam; the white masthead light shows over 225° and the white sternlight over 135°.
Source: Amalgamated International and US Inland Navigation Rules; Convention on the International Regulations for Preventing Collisions at Sea, 1972 (COLREGs)
StandardStrong evidence
Rule 15 requires that when two power-driven vessels are crossing with risk of collision, the vessel that has the other on its own starboard side keeps out of the way.
Source: Amalgamated International and US Inland Navigation Rules
StandardStrong evidence
The regulations' technical annex requires navigation-light chromaticity to fall within areas of the CIE chromaticity diagram defined for each colour.
Source: Amalgamated International and US Inland Navigation Rules

Go further

Red Green Term: Hue Term: Brightness Term: Colour space

Sources

Convention on the International Regulations for Preventing Collisions at Sea, 1972 (COLREGs) — International Maritime Organization (Official standard or government)
Amalgamated International and US Inland Navigation Rules — US Coast Guard Navigation Center (Official standard or government)

Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.

Why do ships show a red light on the left and green on the right?

At a glance

The standard

COLREGs 1972: Convention on the International Regulations for Preventing Collisions at Sea

Arcs, not just colours

Colour tells you who gives way

Colours by chromaticity, not by name

Buoys use colour differently

The data

How to check it yourself

What the evidence says

Related questions

Go further

Sources

Why do ships show a red light on the left and green on the right?

At a glance

The standard

COLREGs 1972: Convention on the International Regulations for Preventing Collisions at Sea

Arcs, not just colours

Colour tells you who gives way

Colours by chromaticity, not by name

Buoys use colour differently

The data

How to check it yourself

What the evidence says

Related questions

Go further

Sources

Why do ships show a red light on the left and green on the right?

At a glance

The standard

COLREGs 1972: Convention on the International Regulations for Preventing Collisions at Sea

Arcs, not just colours

Colour tells you who gives way

Colours by chromaticity, not by name

Buoys use colour differently

The data

How to check it yourself

What the evidence says

Related questions

Related topics

Go further

Sources

Why do ships show a red light on the left and green on the right?

At a glance

The standard

COLREGs 1972: Convention on the International Regulations for Preventing Collisions at Sea

Arcs, not just colours

Colour tells you who gives way

Colours by chromaticity, not by name

Buoys use colour differently

The data

How to check it yourself

What the evidence says

Related questions

Related topics

Go further

Sources