That seems to be typical of fall foliage as the plant begins to shut down for the season. Leaves turn some colors and end browning/drying out. Post again if you notice something else.

Ok, thank you. That is what I was hoping. I know the mother plant had leaves that looked like this in the fall, but I was worried because the change happened so suddenly. I will keep a close eye on them.

Another question- we are expecting our first freezing temps this weekend. Do I need to do something to protect them?

If they've been outside for a few months, they are fully acclimated and can tolerate the cold. In zone 7, I wouldn't necessarily recommend anyone winter-protect hydrangeas to preserve flowering ability, but after last winter, who knows? You might want to at least pile some fallen leaves loosely over them.