Light grey = lean condition
Did it happen suddenly / was fine before?
Probably an intake leak. Or maybe some debris in the fuel supply. Or vacuum in the tank.
Spray the joins in the intake with water or carb cleaner while idling and listen for a change of speed. People often have a leak where the carb pushes on.
If that proves negative then remove the carb bowl and check fuel is freely flowing by letting fuel drain into a jug.
Try with the tank cap tight vs removed.