Question

what causes a stop sign to appear red

Answer 1

Answer:

Red color can be seen from far due its reflection and can detect by human eye any weather condition that’s was why it has been chosen for danger, so human can see and do the needful.

I believe green and yellow has chosen due to its representation in nature, for example green trees etc and yellow is sign of mild danger as we see human face yellow, we start to believe some wrong with person, or may be he sick.