I potted these buddleia up a few months ago from young rooted cuttings. They are not supposed to be yellow. We are thinking perhaps it is the water? We have a well but we have not tested the water. The soil is a mix pine fine, vermiculite and others I can not remember. Nitrogen depletion?
This looks like a nitrogen deficiency. The pine fines does not contain a lot of nutrients. You can fertilize with a liquid fertilizer. Follow label directions. We do see some new green growth.
Please be aware that buddleja is an invasive plant and you may want to take the opportunity to replace with some native species. Here is more about it. https://extension.umd.edu/hgic/topics/butterfly-bush