The answer to 'retaining wall installation turning brown' is that the most common cause is oxidation of the metal components in the wall. As the metal rusts and oxidizes, it can cause the wall to turn a reddish-brown color. This is a common issue with many types of retaining walls, especially those made with steel or other metal materials.
The best solution is to have the retaining wall inspected by a professional landscaping contractor. They can assess the extent of the oxidation and determine if the wall needs to be repaired or replaced. In some cases, the metal components may be able to be cleaned and sealed to prevent further rusting. However, if the oxidation is severe, the entire wall may need to be rebuilt using more rust-resistant materials.