The answer to 'why is my sprinkler system yellowing' is that it is likely due to mineral buildup or a water quality issue. Over time, minerals like iron or sulfur in the water can accumulate in the sprinkler heads, causing them to discolor and become less effective. This can also lead to reduced water pressure and uneven coverage from the sprinklers.
To troubleshoot and resolve this issue, you can try cleaning the sprinkler heads or flushing the system to remove any mineral deposits. If the problem persists, it may be necessary to have a professional inspect the system and make any necessary repairs or adjustments.