Amber lights flash on a school bus to warn drivers that it is about to stop. They are advance-warning signals, telling approaching motorists to slow down and prepare for the bus’s passenger stop.
On the common eight-lamp warning system used in the United States and Canada, amber lights activate while the bus is slowing or approaching a designated stop. Once the bus has stopped and is loading or unloading passengers, red lights normally replace the amber signals, and a stop arm may extend.
The distinction matters: amber does not usually mean that traffic may ignore the bus, but it gives drivers an early warning that a stop is imminent. Red lights indicate the more restrictive stopped-and-loading phase. Exact activation distances and legal requirements vary by jurisdiction.
White and green are not the standard colors for this advance school-bus warning function. Some jurisdictions use different equipment or rules, so local traffic law still controls, but amber is the canonical quiz answer for the question.