There are different reasons that cause yellow leaves and not all of them are a cause to worry.
If the plant is healthy with an active root system, the bottom or older leaves one or two yellowing could be just a part normal aging process of the leaves.
Too much light is another reason for the yellowing of leaves. Too much light will cause the chlorophyll in the leaves to deteriorate which in turn can turn your orchid yellowish to a light greenish colour and eventually cause premature loss of the leaves. If the plant is receiving the appropriate amount of light for a certain period will be a robust light apple green colour.
When a lot of leaves are suddenly turning yellow and drop off or leaves are turning yellow from the plant's pseudobulbs or growing stems, you must check for any pests or for any symptoms of the disease. Bacterial or fungal rots are often the cause, and these are very serious. A bactericide or fungicide treatment is required upon the identification of the disease. In case any pests are found, identifying the pest and pesticide application is required. More often scales or mites are the culprits.
Certain Orchids can be sensitive to an overdose of fertilizer and often evidenced by yellow leaves which eventually dries the leaves and drops. There isn’t much to be done except flushing the potting medium and washing the leaves to remove excess fertilizer.
Nutritional deficiency depending on the orchid you grow also could be a reason. If that’s the case the plant will respond well to a spray of Magnesium sulfate or Epsom salt. Epsom salt or Magnesium sulfate will help the plant to build chlorophyll.
If the roots are not healthy, the plants will be unable to take up water and nutrients. The roots can rot or dry due to overwatering or underwatering. You should check the root system to be sure it is healthy.