There are four colors of passports: green, blue, black and red. Contrary to what one might believe, these do not necessarily correspond to a specific region in the world.
They result from a history, where politics, religion sometimes and a little geography (all the same) mix. Other colors, apart from the four mentioned above, can also be used in passports other than ordinary ones. This is the case for Turkey, which uses gray for its service passport, which is reserved for those who cannot claim a diplomatic passport.