Yellowing leaves on a phalaenopsis are not necessarily a cause for alarm. It's normal and natural for older leaves to yellow and gradually drop off. Older phalaenopsis often have somewhat elongated stems where old leaves have dropped away. On healthy plants, new roots will continuously emerge from the stem, eventually forming a mass of roots.
However, if the leaves are yellowing from the top of the plant, there is a problem. If the leaves are still plump and firm, the plant is most likely receiving too much light and it's washing out the color. If the leaves are wrinkled and listless, the plant is most likely dehydrated.