IEC/EN 60204-1 standards – “Safety of machinery - Electrical equipment of
machines” prescribe the following:

“The colours for START/ON actuators should be white, grey, black or green,
with a preference for white. Red shall not be used.
The colour red shall be used for emergency stop and emergency switching
off actuators.
The colours for STOP/OFF actuators should be black, grey or white, with a
preference for black. Green shall not be used. Red is permitted, but it is
recommended that red is not used near an EMERGENCY operation device.”

Thus, the standard indicates, with a preference but not mandatory, the use of
white for start/on and black for stop/off.
To this day, it continues to be widely prevalent the use of green for start/on
and red for stop/off.
Bear in mind the emergency switching off push button must have a
yellow background immediately around it.