yellow traffic light
Refers specifically to the yellow light in traffic signals.
The light turned yellow at the intersection, so I hit the brakes.
warning sign; sign of danger
Idiomatic use; a metaphorical warning that something may go wrong or is at risk.
The drop in sales is a warning sign for the business.
Compound of 黄 (ki, 'yellow') + 信号 (shingō, 'signal, traffic light').