Certified GKH Gardening Expert
There are a few reasons hydrangea leaves turn purple, which are all covered in this article: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-leaves-turn-purple.htm
My best guess is that the plants are suffering from a phosphorous deficiency, which isn't a big deal. You can apply some phosphorous fertilizer and test your soil's pH (low pH is often the cause of phosphorous deficiency). There's more information about how to do this in the article I posted above.