Why is one garage door sensor yellow and the other green?

One sensor being yellow or amber and the other green is the normal, correct condition for most garage door openers. On LiftMaster, Chamberlain, and Craftsman systems, the yellow or amber sensor is the sender, which constantly emits an infrared beam, and the green sensor is the receiver, which lights when it detects that beam. Both should be solid and steady. The two-color setup makes it easy to tell which side is which during installation and troubleshooting. If your door still will not close with both lights on, the issue is alignment, an obstruction in the path, or a brief beam interruption. If both lights are dim or one is blinking, check the wiring, clean the lenses, and re-aim the receiver.