Maybe it's a response to some unseasonable cold weather or a mineral deficiency. Try giving it a liquid feed of tomato food or a comfrey tea and some slow release clematis food and see if that sorts it out.
Vendée - 20kms from Atlantic coast.
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What variety is yours and does it usually have all purple tepals or is this your first year with flowers?
Maybe it's a response to some unseasonable cold weather or a mineral deficiency. Try giving it a liquid feed of tomato food or a comfrey tea and some slow release clematis food and see if that sorts it out.