Solo uses two types of propellers: silver top and black top. You can find these color markings on the top of each propeller.
Attach the propellers with silver tops to the motors with a silver dot on the top of the motor shaft, and attach the black-top propellers to the motors with black dots.
Be sure to remove the paper labels from the motors before attaching the propellers. The silver-top propellers tighten clockwise; black-top propellers tighten counterclockwise. Check the lock and unlock icons on each propeller to see the correct directions for tightening and removing.