Many Color Tricks
French hydrangeas tend to be blue in acid soils and pink in alkaline soils.
Wendy doesn't try to manipulate the colors of her French hydrangeas. She lets them do what they want. If you want to change or intensify their colors, try these tricks. For blue hydrangeas, dissolve 1 tablespoon of aluminum sulfate in a gallon of water, and drench the soil around the base of plants in March, April, and May. For pink hydrangeas, dissolve 1 tablespoon of hydrated lime in a gallon of water, and soak the roots in April and May. Avoid getting the lime mix on foliage.