The answer to 'retaining wall turning brown' is that a brown discoloration on a retaining wall is often caused by mineral deposits, rust, or organic growth. This can happen over time as water, soil, and weather interact with the wall materials. Common symptoms include a dull, uneven brown color that may spread across the surface.
Some potential solutions include cleaning the wall with a mild detergent or vinegar solution, applying a sealant or water-repellent treatment, and addressing any drainage or moisture issues around the base of the wall. If the discoloration is severe or the wall has structural damage, it's best to consult a professional landscaper or mason to assess the situation and recommend the appropriate repairs.