Safety sensors: sensors located at the bottom corners of door that keep door from closing when door opening is obstructed. Keeping opener from closing door on a car, animal or a person. There are two functions for the LED lights on sensors. Not all sensors have different color lights. The two different colors represent witch sensor is the transmitter and which is the receiver. Also when both lights are lit up with a steady solid light this means both sensors are aligned correctly. When they are not lit up than there is an obstruction or they have been knocked out of alignment.
Some manufactures sensors only have one light on just one of the sensors. And some of the older units from the early 90’s only have one sensor and a reflector on the other side. All still have the same result.
I have seen spider webs with leaves stuck to sensor lenses and can block beam from working properly. The sensors that have only one LED light will usually have weather it is a transmitting or receiving printed on the sensor somewhere. This makes replacement easier for a technician to determine what sensor is not working.