Orchid's leaves became yellow.
Thanks for your question about orchid leaf yellowing. (I know; I experience the same!)
Here's a snippet from this article, which is very thorough, about growing orchids:
Leaves"The leaves on my orchid turned yellow. Should I be concerned?The answer to this question depends upon which leaves are involved. Yellowing of old leaves on backbulbs is a normal part of the aging process. Therefore, this should not be of concern.
However, yellowing of newer leaves is a sign of trouble. Orchids with yellow leaves are an indication that the plant is suffering from either too much light or insufficient feeding. Increase the shading and apply orchid nutrients as directed. Other causes of yellowing include loss of roots and stress due to low temperatures."
That should be lots to digest! Good luck!