What sign is a warning sign?

Answer 1

Under the U.S. Manual of Uniform Traffic Control Devices (MUTCD), adopted by most US states, a "warning sign" may be one of three colors:

Yellow -- indicates an ordinary road hazard ahead (curve, crossroad, merge, etc)

Orange -- used for temporary construction hazards (detour, lane closed, etc)

Pink -- an emergency has created a completely new hazard ahead (fire, collision, police activity ahead).