Thanks for writing. Without being able to see your orchids, I'm sorry to say I can't say for sure what the problem is.
How much do you water them? Orchids are very sensitive to overwatering, especially if they're growing in moss. It's best to let the moss dry out a bit before watering them again.
Do your orchids get exposed to hot or cold drafts? That can also make the leaves go yellow.
Also: Orchid leaves do start to grow old and die. Most of the time, an individual leaf will stay green for a couple of years before it succumbs to old age and is replaced by new leaves.
Lastly, make sure your orchids don't need repotting. Many common orchid mixes last a couple of years before they start to break down. When they do and they start to look like soil, they're bad for your orchid. If this is the case, repotting it in fresh bark should help a lot.
Thanks for writing. I hope this helps!
---Justin, Senior Garden Editor, BHG.com