The use of red on signs is limited to stop, yield, and prohibition signs. A white background indicates a regulatory sign; yellow conveys a general warning message; green shows permitted traffic movements or directional guidance; fluorescent yellow/green indicates pedestrian crossings and school zones; orange is used for warning and guidance in roadway work zones; coral is used for incident management signs;** blue indicates road user services, tourist information, and evacuation routes; and brown is for guidance to sites of public recreation or cultural interest.
https://mutcd.fhwa.dot.gov/services/publications/fhwaop02084/index.htm