I have several Phalaenopsis orchids which flower pretty well in general. However, I have one which has just flowered but the leaves are gradually turning yellow and falling. There is now only one left.
Phalaenopsis Orchid Leaf Loss
Certified GKH Gardening Expert
Older leaves that are yellowing are the normal process of the plant aging.
This is no concern and doesn't require any changes.
If the new leaves are yellowing, this is a sign of trouble.
This could be to much light or improper feeding. Increase the light and add orchid fertilizer as directed on the package.
Also be mindful of temperatures, as an orchid that is to cold will show stress with yellowing leaves.
Over watering can also lead to root rot and root loss. Make sure the orchid is properly potted and watered.
Here are some links to refresh you on the care requirements.