Inlet Grille inside Dryer is Discolored
You may see some discoloration on the rear dryer grille where hot air comes into the dryer.
This is normal and is caused by a number of factors, including usage, color transfer from clothing, and heat. This discoloration is permanent, and will not wear off on any other clothing or material.
See pictures below for a new dryer grille, then one showing this discoloration:
New inlet grille
Discolored inlet grille (normal)