noun
yellow traffic light
Refers specifically to the yellow light in traffic signals.
交差点で黄信号になったので、ブレーキを踏んだ。
The light turned yellow at the intersection, so I hit the brakes.
noun
warning sign; sign of danger
Idiomatic use; a metaphorical warning that something may go wrong or is at risk.
売り上げの減少は、経営の黄信号だ。
The drop in sales is a warning sign for the business.
Green traffic light; metaphorically a go-ahead or safe sign.
Compound of 黄 (ki, 'yellow') + 信号 (shingō, 'signal, traffic light').