In the United States, what color are warning signs for school areas normally?

In the United States, warning signs for school areas are normally fluorescent yellow-green.

The high-visibility fluorescent yellow-green colour is used for school-zone and school-crossing warning signs because it stands out strongly against many road and landscape backgrounds. The Manual on Uniform Traffic Control Devices permits this colour for signs associated with schools, school crossings, and some pedestrian or bicycle warnings. Traditional yellow remains the standard background for most permanent warning signs.

The colour does not by itself create a speed limit or define the exact legal hours of a school zone. Drivers must read the sign legend and any supplementary plaque, which may state a reduced speed, crossing location, or time-based restriction.

Fluorescent yellow-green is sometimes confused with ordinary yellow. The fluorescent version is brighter and more conspicuous, especially in daylight. It is a sign-background colour, not a universal colour for every road marking near a school.