Personal Passport (blue cover)
Also called “tourist passports,” “regular passports,” and “ordinary passports,” the passport book with the blue cover is the one issued to the vast majority of US citizens. It is appropriate for all travel purposes except for travel for US government purposes, including tourist travel, business travel, flight crew travel, and travel for education or employment.
Official Passport (brown cover)
Official passports are issued to US government employees and contractors who are traveling on US government business. They are also issued to active-duty members of the military who will travel overseas on military duty. Official passports are issued only in Washington, DC, at the Special Issuance Agency of the Washington Passport Agency.
US citizens may hold both a valid official passport and a valid personal passport at the same time. Official passports are only to be used when traveling on government business, so any leisure travel must be done with a personal passport.
https://passportinfo.com/us-passport-help-guide/passport-books/