Red, blue, green and black dominate passport covers, so the internet turned an observation into a “rule.” But there is no international regulation saying ordinary passports may use only four colors.
What is standardized is far more practical: machine readability, document dimensions, data layout and security features.
The inside matters more than the cover
ICAO Doc 9303 sets technical specifications for machine-readable travel documents. Those standards help passports work across borders and inspection systems. A global four-color palette is not the point of the document.
Tradition, identity and practical design
Red and burgundy
Common across Europe and elsewhere, with regional and historical associations but no universal political code.
Blue
Used by countries on several continents, including many passports in the Americas.
Green
Often associated with national or religious symbolism in some states, but again not by international requirement.
Black and very dark shades
Dark covers age well visually, offer strong contrast for metallic crests and are used by several countries.
“Color” is not four exact paint chips
Passports appear in burgundy, maroon, navy, royal blue, teal-green, charcoal and other variations. Some countries also change colors between ordinary, diplomatic, service and emergency documents. ICAO discussions on emergency travel documents explicitly recognize that states may choose a different cover color to distinguish them.
Mostly national design choices
Coats of arms, typography, biometric symbols, language, material and color together form the visual identity. The cover is cultural branding wrapped around an internationally interoperable document.
Four dominant color families, not a four-color law
That version is less mysterious — and more accurate. Passport design is constrained by security and interoperability, but governments retain wide visual freedom.