The answer may surprise you.

Passport Four Colors
Credit: REUTERS / Alamy Stock Photo

It’s often assumed that passport covers can only be made in shades of blue, black, green, and red. And generally speaking, it’s pretty much true that countries opt for these dark, official-looking hues.

But it turns out that there’s no official regulation forcing these countries to select black and primary colors.

“Any color that’s in the Pantone book, we can make,” William Waldron, the vice president of security products at Holliston, LLC (which makes passports for more than 60 countries) told Travel + Leisure.

That means that if the United States wanted to print passport covers in the Pantone Color of the Year, 15-0343 (or Greenery, as it’s more commonly known), they could.

According to the International Civil Aviation Organization, passports (or any machine-readable official travel document) must be made of a material that bends, rather than creases. They must be stable in temperatures between 14 and 122 degrees Fahrenheit, and should remain readable in humidity conditions between 5 and 95 percent.

There are, however, no stipulations in place that dictate how a passport looks. While the ICAO makes suggestions about the typeface, type size, and font, even the details are at the discretion of the issuing state — though they strongly suggest printing information in upper-case characters.

“Nothing stipulates the cover color,” confirmed Anthony Philbin, ICAO’s chief communications officer.

So why the global predilection to navy blues, maroons, deep forest greens, and black? As we’ve discussed previously, geopolitics and religion certainly come into play when a country determines the color of their passport. Muslim countries, for example, largely prefer green passports, because the hue is so significant to the religion. And Caribbean states typically opt for blue passport covers.

More practical reasons can also influence a country’s decision to opt for predictable, dark tones. Waldron says they are “generally more official looking” and “less likely to show dirt and wear.’

Mystery solved.