Passports are only made in shades of blue, red, green, and black.

You might assume that this is because of regulations governing which colors passports can be, but there's actually no official rule dictating acceptable passport color. In fact, there are no rules about what passports should look like at all, only suggestions. The International Civil Aviation Organization (ICAO) makes recommendations about typeface, type size, and font in their guide for machine readable travel documents (MRTDs), a category which includes passports.

There are some hard and fast regulations in this document, however: Passports must be made of a material that bends (no creasing) and remain machine readable at temperatures ranging from 14 to 122°F and at relative air humidity ranging from five to 95 percent.

Although passports only come in these four colors, there's a lot of variation when it comes to the actual shade. Many countries opt for darker shades of blue, red, and green. The reason for the limited color choice is pretty much what you'd think: Countries choose these simple, dark hues because they look official. Millennial pink wouldn't look very bureaucratic. Dark colors are also less likely to show dirt and wear. (You can browse the world's passports with this nifty website.)

Anthony Philbin, ICAO’s chief communications officer, confirmed to Travel + Leisure that when it comes to passports, “Nothing stipulates the cover color.” Basically, there's nothing stopping the United States from making its passports lime green except a sense of propriety. William Waldron, the vice president of security products at Holliston, LLC (which makes passports for more than 60 countries) told Travel + Leisure that they can manufacture “any color that’s in the Pantone book.”

There's often meaning behind which of the four colors a country chooses. For example, most Islamic states use green passports because the color is significant in their religion. Though the majority of passports issued in the U.S. today are blue, that change came about in 1976, as part of the bicentennial celebration; prior to that, from 1941 to 1976, American passports were green—a color that made a triumphant return between 1993 and 1994 and included a special tribute to Benjamin Franklin.

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