When you see yellow leaves on a plant, you know there's a problem. Unfortunately, this is the upset stomach of gardening -- yellow leaves represent any number of conditions, including the following:
If you see yellow leaves, look at the plant's environment and see if you can detect one of the above problems. Correcting that may easily correct the yellowness. In some cases, such as temperature-related color change, the problem may have already corrected itself. In others, such as aging foliage, you need not do anything, because it's part of the plant's natural cycle of life.