Here are a couple of tricks to turn hydrangea flowers the color you want. To turn blue flowers pink, sprinkle 3 to 4 cups of lime around the base of the plant. Do this again several months later. To turn pink flowers blue, add 4 tablespoons of aluminum sulfate to a gallon of water, and drench around the base. Do this again three weeks later. The color change takes about a year in either case, so be patient.
What about altering a white hydrangea? Some people claim there is no way to change the color, but I disagree. Just stop watering it for a few months at the height of summer and watch what happens.
"Hydrangea Color" is from the June 2002 issue of Southern Living.