What's the colour of your passport?

My husband just told me that his daughter is renewing her passport in Spain, for her trip this Tuesday. She's coming here to have a vacation. And speaking of passport.... It makes me wonder, why do we have different passport colour? When am in the airport, I always look at other people's passport and the colour, wondering where they're from or what's their nationality. I asked my husband, he said he doesn't know. Who chooses the colour for each contry? Is it assigned? What's the colour of your passport? Mine is green.