The colors vary between blue, red, green and black,
The red is used by member countries of the European Union, except Croatia, and countries which didn’t join yet, such as Turkey.
The block of South American countries, including, Brazil, Argentina, and Paraguay, also use the blue passport, the cover symbolizing the connection with Mercosur.
The Green color used by the majority of Muslim countries, due to the importance of the color in their religion, as green is considered to be the favorite color of the Prophet Muhammad. It is also a symbol of nature and life.