The answer to 'why is my drainage system yellowing' is that the yellowing is typically caused by mineral buildup or bacterial growth in the drainage pipes. This can happen over time as water flows through the system, leaving behind deposits that discolor the pipes.
Some common symptoms include slow draining, standing water, and a foul odor coming from the drainage system. To fix this, you'll need to clean out the pipes by snaking, jetting, or replacing the affected sections. In some cases, the entire drainage system may need to be replaced if the issue is widespread.