The answer to 'mulch yellowing' is that it is a common issue caused by a lack of nitrogen in the mulch. This can happen when the mulch is not properly composted or if it is made from materials that are low in nitrogen, like wood chips or shredded bark. The yellowing indicates that the mulch is breaking down and not providing enough nutrients for your plants.
To fix this, you can try adding a nitrogen-rich fertilizer to the mulch, like blood meal or composted manure. You can also try mixing in some fresh, nutrient-rich compost to help balance the nutrients. If the yellowing persists, it may be time to replace the mulch entirely. In some cases, you may need to call a professional landscaper to assess the issue and provide a more comprehensive solution.