The answer to 'air duct discolored' is that discoloration in air ducts is often caused by a buildup of dust, debris, and microbial growth. This can happen over time as the ducts circulate air throughout your home. Discolored ducts can be a sign of poor indoor air quality and may indicate the need for professional duct cleaning.
Common symptoms of discolored air ducts include visible staining or darkening of the duct surfaces, musty odors, and an increase in allergy or asthma symptoms for occupants. Solutions can include regular professional duct cleaning, sealing any air leaks, and upgrading your HVAC air filters to higher efficiency models.