The answer to 'why is my drainage system yellowing' is that the yellowing is likely caused by a buildup of iron oxide or sulfur compounds in the drainage pipes. This can happen when the pipes are made of certain materials like galvanized steel or when the soil around the pipes contains high levels of these minerals.
Some common symptoms of this issue include yellow or orange discoloration in the water coming from the drainage system, as well as staining or residue left behind. To fix the problem, you may need to clean out the pipes, replace any corroded sections, or treat the soil around the pipes to remove the mineral buildup.