If you’re growing Hydrangea Marcrophylla (French Hydrangea) you can turn it from pink to blue by lowering the pH level of your soil. The colour is determined by the amount of aluminium available to the plant. Add 3 to 4 tablespoons of aluminium sulphate to a gallon of water. Apply it 2 times, 2 weeks apart, for 4 to 6 months prior to blooming. You can also add coffee grounds, eggshells, lemon juice or ground-up citrus peel to the soil. You’re trying for a pH of 5-6.
On a similar note, blue hydrangeas can be changed to pink by adding 3-8 cups (depending on the size of the plant) of dolomic lime around the base of the hydrangea. Do this 2 or 3 times between the blooming seasons. As an alternative, you can also try a bit of superphosphate around the base of the plant. Go for a pH of 6.8-7.2.
The colour change doesn’t happen overnight so be patient. It can take as long as a year or two to see the results. The white varieties of hydrangea (such as PeeGee, Silver Leaf, Hills of Snow, etc.) cannot be changed. They remain white regardless of tampering with the pH of the soil.