How do I change the color of my hydrangea?

Flower color in H. macrophylla is dependent on cultivar and aluminum availability. Aluminum is necessary to produce the blue pigment for which bigleaf hydrangea is noted. Most garden soils have adequate aluminum, but the aluminum will not be available to the plant if the soil pH is high. For most bigleaf hydrangea cultivars, blue flowers will be produced in acidic soil (pH 5.5 and lower), whereas neutral to alkaline soils (pH 6.5 and higher) will usually produce pink flowers. Between pH 5.5 and pH 6.5, the flowers will be purple (see image at left) or a mixture of blue and pink flowers will be found on the same plant.

It isn't unusual to find inflorescences that are
          part pink and part blue (see image at right).

Source:  http://www.usna.usda.gov/Gardens/faqs/hydrangeafaq2.html

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