I have not yet tried to grow a hydrangea hydroponically, but a fascinating study done at Okayama University proves that although people have long controlled the color of the hydrangea blossoms by adjusting the soil pH, the same result could be had in hydroponically grown hydrangeas without changing the pH. They adjusted Al (aluminum) instead.
A dear neighbor gave me a most gorgeous blue hydrangea from the flower shop and after the flowers wilted I cut back the stem and waited for new growth:
