Applies to PM45 printers that will print on long color labels such as Burnt Ends #21016:
Label sensor is a blue light under the label just before the printhead.
It can be adjusted back and forth via a spinning wheel to the right and below the printhead.
To adjust for long colored labels like 21016, use the adjustment wheel to move the blue light all the way to the left when looking directly at the front of the printer: