To turn a hydrangea to blue, aluminum must be present in the soil around the plant. To ensure that aluminum is present, Aluminum Sulfate may be added to the soil around the hydrangeas.
Experts recommend that a solution of 1/2 oz (1 Tbsp) aluminum sulfate per gallon of water be applied to plants (which are at least 2-3 years old) throughout the season. Important: Water plants very well in advance of application and put solution on conservatively, as too much can burn the roots of hydrangeas.
To make the aluminum available to the plant, the pH of the soil should be low/ acidic (5.2-5.5). Adding aluminum sulfate will tend to lower the pH of the soil. Another method for lowering the pH is to add organic matter to the soil such as coffee grounds, fruit and vegetable peels, grass clippings, pine needles, sulphur etc.
If the soil naturally contains aluminum and is acid (low pH) the color of the hydrangea will naturally tend toward shades of blue and/or purple.
To change a hydrangeas color to pink, you must remove aluminum from the soil, or make it unavailable to the plant by raising the ph. Try using granular Dolomitic Limestone Pellets around the plant to "sweeten" soil and make aluminum less available. This should be done a couple of times per season. Ph levels should be in the 6.0 range. Using high phosporous fertilizers also help change the color into the pink range. Phosphorous is the middle number in the fertilizer analysis.