Thanks for your question.

The appearance of a green appearance in coneflowers, whose flowers may otherwise be purple, is a good indicator of an aster yellow infection. In your pictures, some of the flowers seem distorted. This, too, indicates the presence of aster yellow. As you may or may not know, aster yellow is caused by a tiny organism called a phytoplasma, which is similar to a bacterium. The phytoplasma is carried from an infected plant to an uninfected plant by aster leafhoppers, which are small insects that feed upon plant juices. There is no good way to destroy this leafhopper. Pesticides are ineffective. There also is no way to cure a plant once it is infected. The only reasonable course of action is to prevent its spread via leafhoppers. Your infected coneflowers need to be removed from your garden and composted. This disease can appear in other annuals and perennials so be on the lookout for symptoms in such plants. Once there is any indication of infection elsewhere, those plants should also be removed and composted. This is why as a gardener you need to be familiar with the symptoms of infection. The one bright spot is that this carrier insect does not live in the soil. Consequently replanting in the same area where the infected plants had been is possible. See the following for information on these and related points:

https://extension.umn.edu/plant-diseases/aster-yellows

https://hortnews.extension.iastate.edu/aster-yellows

https://www.thespruce.com/how-to-identify-aster-yellows-4781045

Aster yellow is the bane of a gardener’s existence. It can, however, be dealt with through effort and diligence. Good Luck.