Certified GKH Gardening Expert
Yellowing leaves on an orchid can be due to several factors.
Bottom leaves that are older can just be aging and will turn yellow and fade.
Another cause can be to much light; light is important but the leaves can easily be burned from to much light.
To much water can lead to the roots rotting and leaves will turn yellow.
Make sure the orchid is not over watered.
If the temperatures are to cold the leaves may turn yellow. Temperatures should be between 65 and 80 degrees F.
Here is a link to refresh you on the care requirements.