Passports for all the countries have been the same size (15.5cm X 10.5 cm) since 1920, and the International Civil Aviation Organization (ICAO) has regulationsAnd recommendations on the typeface, type size, and font, etc. on the material of the passport: a) it should bend without creasing, b) it should remain machine-readable at temperatures between -10oC to 50oC & relative humidity of 5% to 95%.
But there are no official rules governing what colours, the passports must be.
Still, the passports around the world come in shades of only 4 colours: Blue, Red, Green and Black (within each colour, there are vast variations).
One of the primary reasons for the four-colours limitation is that passport production is a highly controlled process, and many security features go into a passport.
Secure printing is not easy and the special outer cover that can support the required security features is usually supplied by a third party and it only comes in certain colour variations.
Other reasons that have been stated are a) these simple colours look official and b) they are better at hiding signs of dirt and wear.
It is believed that there is often a meaning behind which of the four colours a country chooses, e.g., most of the Islamic countries have green-coloured passports because green holds significance in the Islamic religion.
But there are too many exceptions for this to be true; a number of countries with a majority Islamic population such as Azerbaijan, Bahrain, Brunei, and Iran, etc. have passports that are not green.
Alliances and aspirations are the other two factors that are said to play a role.
E.g., most of the European Union’s member countries and countries with the historical or current communist system (China, Serbia, Russia, Latvia, etc.) have red-shade passports; countries interested in joining the EU(Turkey, Macedonia, and Albania) also changed the colour of their passports to red a few years back.
As per Passport Index, blue is the most common shade (82 countries), followed by red (66) and green (44).
Black passports are the rarest (only 7 countries).
With the advancement in technology (outer cover in different shades supporting security features) and an increasing need to ‘stand out’, it may not be long before countries come up with colours and designs that define the country’s personality.
The Norwegian government took a lead on this front when it set up a national competition for passport design in 2014-15; you can have a look at the winning design here.
Very well explained.