This one is tough to properly diagnose because it can be one of two things that we cannot easily (or cheaply) diagnose.
Winter damage to leaves can cause opportunistic fungal infections that develop when the winter injuries in the leaves get wet often. Treat it as if you had a case of leaf spots.... Never water the leaves. Instead, water the soil early in the morning. Allow separation between plants to increase air flow and reduce the chance of fungal problems. Do not overwater and do not let the mulch stay wet for long periods of time. Prune dead stems and pick up any plant debris. Dead or dried out blooms/leaves should be disposed in the trash.
In addition to fungal infections, the weather can also cause some hydrangea leaves to turn these reddish/purplish colors early. Some varieties that produce red-purple colors in Fall may develop them early when the weather cooperates. If this is the case, you do not need to take any action.
To correctly diagnose, you would need to send a leaf sample in a sealed transparent plastic envelope to your Agriculture Extension Service or a university. Or just follow the good sanitation suggestions given for fungal problems... no need to apply fungicide.