The American National Standards Institute  (ANSI) makes the following requirements for low energy power operated pedestrian doors:

All low energy doors shall be marked with signage visible from both sides of the door, with the words "Automatic Caution Door".  The sign shall be a minimum of 6 inches (152mm) in diameter with black lettering on a yellow background.  See picture below.  Additionally, when a Knowing Act Switch is used to initiate the operation of the door operator, the doors shall be provided with signs on each side of the door where the switch is located, with the message "Activate Switch to Operate".  The lettering shall be white and the background shall be blue.