There are lots of reasons why orchid leaves turn yellow. The biggest culprit is usually either a bacteria or a fungus. Bacterial infections can cause yellow spots on leaves. You'll need to get a bacteriacide to kill the disease, if that's what it turns out to be. If there's a black ring around the yellow spot, the disease is most likely a fungus and you'll need a good fungicide.

Too much water and light can also cause leaves to turn yellow. Overwatering may also cause the roots to rot so if you suspect that too much water is the problem, be sure to check the roots as well as the leaves. If the roots are black and mushy you'll have to trim them off. You can slow down on the watering and make sure your orchid is drying out in between watering and that it's not potted in a medium or a pot that's causing water to be held too long around the roots. If there's a fungal infection, as evidence by the black spots we mentioned, that may have been encouraged by too much watering or water sitting on the leaves for too long after misting.

If disease and water don't seem to be the problem, the problem is most probably too much light.  Pull your orchid out of direct light. The yellowed leaves won't turn green but none of the other leaves should turn yellow. If more leaves continue to turn yellow forge on and figure out the problem.

Have you subjected your orchid to temperatures that are too low for it? Check the low temperature limits of your orchid and see. When it's too cold your orchid might develop yellow spots on its leaves.

When Phalaenopsis orchids produce new growth, the leaves on the bottom of the plant usually fall off. Dendrobiums often drop some leaves after they're finished blooming. Then of course, you have to keep in mind that leaves, just like all other living things, get old and die. If the yellow leaves are towards the bottom of the plant they may just eventually fall off as new ones grow on the top. If the leaves are towards the top of the plant and the bottom leaves are fine, old age is not the problem.

So yellowing of your orchid's leaves may be totally natural or it may signal a problem that your orchid is having with either a disease or something related to light, water or temperature. Whatever the problem, always keep a good eye on your plants and take a look at them every day. The easiest problems to solve are often the ones that are caught early.