The soil appears dry in the picture so maybe inconsistent or insufficient watering. Remember that their root system is still small and may have been disturbed when transplanting. When planted in the middle of the summer, hydrangeas react strongly by wilting leaves, wilting stems, browning-yellowing blooms, etc.
Consistent watering: Aim to keep the soil evenly moist at a depth of 4", the typical depth of most newly planted hydrangeas. Immediately as the soil at that depth turns almost dry or dry, water deeply.
Sufficient watering: Use enough water so the soil will feel moist after watering and waiting for the soil to percolate. But do not water on your watering day if the soil is already soggy.
Maintain the soil always mulched with 3-4"past the estimated width of your plants at maturity (about 10+ feet) to minimize soil moisture loss.
Provide supplemental afternoon shade using sheets, 35% shade cloth, large umbrellas, etc.
Note: some leaf yellowing is common with panicles like Limelight as they get summer heat stressed (the leaves near the crown/base of the stems turns completely yellow and drops). Use that as an indication that you need to increase the amount of water from spring watering levels to summer watering levels.
If the blooms are exposed to temperatures near or above 100F, they may prematurely brown out even if the shrub is properly watered as the blooms are not "designed" to handle that type of heat. Here in Texas, Limelights requires supplemental shade by 3pm and lots of watering. The blooms often brown out after days or just a few weeks.
When planted in the middle of the landscape by themselves, the shrubs are also affected by winds that can easily desiccate the leaves so it is imperative to keep the small rot system evenly moist until it grows larger and is not so reactive to the heat.
