Some hydrangeas -- most macrophylla and serratas -- do change colors! In some varieties of these pink-to-blue hydrangeas, like Tellers Blue or Twist n Shout hydrangeas, the soil pH will affect the color of the flowers. These flowers become more blue in highly acidic soils, and lilac to pink in slightly acidic to alkaline soils. To make your hydrangea flowers more pink, you'll need to raise the soil pH, making the soil more alkaline and less acidic. Try adding lime to the soil will make the flowers appear more pink. Dolomitic limestone will also add magnesium to the soil. Wood ash will also raise soil pH, but should be applied very lightly: too much can burn your hydrangea's foliage. Lowered soil pH is often associated with hydrangeas of a bluer hue, but low pH won't make your hydrangeas blue on its own. Aluminium is the key to turning hydrangeas from pink to blue, and an acidifier without aluminum in it may not achieve the same effect. Aluminum availability in the soil is pretty common, but not universal, so look for amendments that include aluminum. In overly alkaline soils the aluminum ions are locked up with other elements, and don't flow to your plant. Lowering the pH allows those aluminum ions to break free and be taken up by the plant, producing the blue color.