It's impressive that the world's 199 countries have all agreed on a standardized design for the passport: A single color, and the country's name and crest written in gold metallic ink.

However, not all passports are equal. The Passport Index ranks them from most powerful (United Arab Emirates) to least (Afghanistan). The determining factor is the Mobility Score, which is based on the number of countries you can enter with the least visa hassles.

As it turns out, the colors also have an informal significance, as the Passport Index writes in a report:

"Blue passports often symbolize the 'new world,' free markets and the ocean. Red covers tend to be associated with Communist countries or Christian history, while green is often the color of choice among Muslim countries because it has religious significance."

"As the rarest color, black tends to symbolize local culture or politics and, on a practical level, hides wear and tear."

The number of countries that have chosen each color for their passports is as follows:

Blue – 83

Red – 67

Green – 42

Black - 7

If you want to see what the world's different passports look like, or where your country ranks in the line-up, you can check it out here.