The answer to 'landscape lighting turning brown' is that the most common cause is oxidation or corrosion of the metal fixtures. Over time, the metal components can develop a brown or rusty appearance due to exposure to the elements. This is a common issue with landscape lighting, especially with cheaper or lower-quality fixtures.
To fix the problem, you may be able to clean and restore the fixtures yourself, but in some cases, the fixtures may need to be replaced. If the issue is widespread or the fixtures are heavily damaged, it's best to call a professional landscaper or electrician to inspect and replace the lighting as needed.