https://mutcd.fhwa.dot.gov/services/publications/fhwaop02084/index.htm

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.

Why bother remembering when you can just look it up in the MUTCD?