The color will change as soon as you change the pH of the soil which will happen if you give it some acid fert like aluminum sulphate.
Hydrangeas like to grow in similar conditions to azaleas, acid soil, lots of humus, moisture and cool shade. Your plant may be distressed just because it is no longer surrounded by its friends - the other pots around it were probably keeping the roots cooler. Try giving it a cool drink with the hose in the heat of the afternoon.
Yes, I think the burnt leaves were from the sun exposure. They won't recover but as long as parts of them are green they're still working for your plant.
