Despite the variety of colors available, most people associate French hydrangeas with the classic blue hue. Sometimes, the hydrangea you bought as blue changes color and blooms pink the following year. Your hydrangea is simply responding to the soil pH. Blue flowers are produced in acid soil (pH 5.5 and lower), and pink flowers are produced in alkaline soil (pH 7 and higher).
You can add aluminum sulfate around your hydrangeas to acidify the soil. Add lime to make your soil more alkaline. Some selections are less affected by soil pH than others. White-flowering ones, such as ‘Lanarth White’ and ‘Madame Emile Mouillere,’ will stay white.