To Make Them Blue: The color of hydrangeas depends on the presence of aluminum and the acidity of the soil. In Georgia, the pH of the soil is usually low enough (pH 5.5 or lower) to allow plants to take up enough aluminum to produce beautiful blue flowers. To keep them blue, you can add aluminum sulfate to the soil. Use the sulfate sparingly, follow label directions, and water well before and after the application.
To Make Them Pink: Pink flowers require soil with a higher pH than is normally found in Georgia (pH of 6.5 and higher). To raise the pH, add dolomitic lime to the soil around the plant several times a year. You can also use a fertilizer with high phosphorous (the second number, such as 25-10-10). Or you can control the soil conditions by planting your hydrangeas in containers and using bagged soil.
Where to Plant: In warmer climates Hydrangeas do well in a partly-shaded location with moist, well-drained soil. Some varieties, like Panicle hydrangea, are more tolerant of direct sun than others. Too much shade and your plants may not flower at all.
Resources: The United States National Arboretum has some great information on Hydrangeas: http://www.usna.usda.gov/Gardens/faqs/hydrangeafaq2.html#How-change-flower-color