The flowers on any flowering shrub are merely a temporary display - if these are the same flowers which were present when you got them, its time they disappeared because they're over, and that process starts with browning.

Seems a bit quick for them to brown to me, though. Plfearn, even though they were watered regularly, did they wilt between waterings? High heat will also accelerate the flower's aging, too.

Tug, if the plants were purchased actually in flower, those flowers could have been present for at least 2 weeks or more at point of sale, who knows. Maybe more will open... but if they're in hot sun, specially in the middle of the day, that might cause this.

Given the way a lot of GCs treat there plants if the blooms survived two weeks after purchase you are doing well… If they were from Lidl extremely well!

Did you water at the base of the plant or drop the water in from the top?

I always look to buy hydrangeas which are not in flower and are named varieties as many are grown abroad under glass and forced into flower robbing the plant of a good stem and root structure. As suggested the flowering might well be over and I would cut off the old flowers as it's still early in the season.