Newly planted hydrangeas will suffer wilting episodes often as we approach the summer months. But established hydrangeas recover on their own by the next morning, provided that the soil remains moist and they are well mulched (about 3-4" of mulch past the drip line). If you see them wilting, use the finger method to check the soil moisture. If the wilting episode seems extreme or if you detect that the soil is drying out, water them immediately (1/2 gallon of water).
Another problem that I have here in the Dalls/Fort Worth Area is sun exposure. If I let the leaves get sun past 11am-12pm approximately, the leaves will suffer from sunscorch by the middle of summer. I mention this because you said that your hydrangeas get sun until mid afternoon. So keep an eye on the soil moisture but check the leaves too. If the leaves start turning all yellow -including the leaf veins- or if they turn somewhat white-ish, you will need to transplant the hydrangea to a more shaded locale or erect something that will provide shade earlier in the day. In April and May, the amount of sun is going to be fine but monitor frequently from mid-May thru September. The hottest part of the year in Austin should befrom mid-July, thru August and parts of September (it varies year by year).
About the finger method.... Water the shrub about 1 gallon of water when a finger inserted to a depth of 4" feels dry or almost dry. Do this daily for 1-2 weeks. Every time that you water, make a note in a wall calendar. Then, after the 2 weeks, observe how often you watered. Then set your sprinkler or drip irrigation to water early in the mornings 1 gallon of water on the same frequency (every 3/4/5 days). As the summer months approach, recheck using the finger method if the temperatures have changed by 10-15 degrees and stay there.
After a year, your hydrangea will be established and will be better able to withstand the summer weather. It will still wilt some but, it will do so less and less every year. But.... there is no healthy way to completely stop wilting when the temperatures are up in the 100s.
Wilting occurs when the leaves loose mositure faster than the roots can absorb it. Wilting reduces the surface area that the sun strikes and allows the leaves to loose less moisture. As the amount of moisture lost and gained equalize at sundown or in the evening, the leaves begin to recover.
As for the pink blooms changing color, it may be lack of moisture or it may be the normal changing of color that hydrangea blooms undergo as they mature. A picture might help.