The answer to 'why is my air duct discolored' is that discoloration in air ducts is often caused by a buildup of dust, dirt, and debris. This can happen over time as the HVAC system circulates air through the ductwork, carrying particles that stick to the interior surfaces. Discoloration may also be a sign of mold or mildew growth, which can occur if there is excess moisture in the system.
In most cases, discolored air ducts are not a major health concern, but they can indicate that the ductwork needs cleaning. Regular professional duct cleaning every 3-5 years is recommended to remove built-up contaminants and maintain good indoor air quality. However, if you notice significant mold growth or the discoloration is accompanied by musty odors, it's best to call in a professional to inspect and address the issue.