Why have some leaves on my orchid turned yellow!
Certified GKH Gardening Expert
This is most usually caused by a lack of fertilization. You can use specially made orchid fertilizers. Mix at half strength and feed once a week with your regular watering.
Here is an article for more information: https://www.gardeningknowhow.com/ornamental/flowers/orchids/tips-on-fertilizing-orchids.htm