There are no strict restrictions dictating passport color, though there are guidelines. Geography, politics and even religion plays a vital role when a country selects its passport color.
Burgundy passports are used by members of the European Union and countries interested in joining have changed their passport colors to match.
The Swiss passport matches the color of their flag.
Pakistan and many Islamic countries use green color because of the importance of the color in their religion.
New Zealand’s is black because it is one of its national colors.
US has changed colour of their passport many times over the years. American passports were of navy blue in 1976, to match the flag. The US passport is going to get another face lift & the new design is yet to be released soon.