There is no required color for emergency vehicles in the US. There may be state requirements in some places which require specific colors or specific light colors. In general each squad/ or local government is able to choose their colors. Around here (Near Chicago), most fire trucks are still various shades of red, ambulances vary from yellow, to blue, orange, red, and white, and police cars can be just about any color whether marked or unmarked. Lights for Firetrucks and EMS are typically either only red, or red and blue and Police are typically red and blue except in Chicago proper where they are only blue.