The answer to 'mulch turning brown' is that this is a common issue with mulch, often caused by decomposition or environmental factors. Mulch naturally breaks down over time, which can cause it to change color from its original dark brown or black to a lighter, more faded brown.
Some common causes of mulch turning brown include exposure to sunlight, lack of moisture, and the natural aging process of the organic materials in the mulch. This is not necessarily a sign of a problem, but it may indicate that it's time to refresh or replenish the mulch in your landscaping.