Certified GKH Gardening Expert
Yellowing is caused by many factors. Before adding anything, you will need to test the soil to make sure that what you are adding is deficient, as adding too much of a nutrients can cause more issues.
These plants are VERY particular in their care and environment, and any deviation from their natural care will cause multiple symptoms.
This article will explain the care of these picky plants. Once that care is provided, perfectly, then you can begin to diagnose issues outside of that:
It is worth noting that they are not near hardy in your zone, so wintering them indoors will be necessary.