You probably have a mix of things going on. First, be aware that, except for limited periods of time, hydrangeas do not do well inside the house. The inside of the house during winter and summer tends to get dry and they do not like that.
Planting them outside in the summer is another example of stress for these water loving plants too. But similar stress can happen inside the home in the Summer. Transplant shock is another issue with all plants when you bring them home from a coddled nursery location. The plants probably do well in Spring (or for a few weeks in Summer) but then they get stressed in the summer as soil moisture goes down and the sun is stronger. This stress can make some old leaves turn all yellow -including the leaf veins- and then drop. Stress from insufficient soil moisture can make the leaves brown out from the edges inwards. They can also suffer from a sun exposure that is longer than they had before at the plant nursery/grocery store/etc and turn all yellow, including the leaf veins. You can also see leaf wilting if the area is windy (think: air conditioner blowing air at them all day long) and they do not have sufficient water.
I would not spray mist hydrangea leaves because that can encourage development of fungal issues like powdery mildew.
If the number of affected leaves is not large in proprotion to the size of the bush, I would just monitor the sun exposure (a few hours fo morning sun), monitor the soil moisture, check air from a/c vents (move the shrub if it gets direct air) and make sure that you have some mulch to make the potting mix stay moist longer in the potted environment. If a finger inserted into the potting mix to a depth of 3" (2.5 cm) feels dry, water it again. Probably best to water until you see water seeping out; stop watering; when the water stops coming out, add the remaining water. On a shrub that is planted on the ground, 1 gallon of water (about 3.8 liters) per watering is sufficient.
Many hydrangeas sold at grocery stores or florists are meant for short term use. Basically, the companies want you to enjoy the blooms and then dispose of the plants. Some of them are not winter hardy varieties but some are. Unfortunately, they do not advertise the variety's name so one is left at a loss as to 'can I plant it outside?' and 'will it survive my winters?' In my climate zone 8, they can probably grow well outdoors but in cooler zones, they may struggle. A Hydrangea macrophylla like yours can withstand winters as cold as Zone 6 or 7.