Naturally creamy white to pale green in color, hydrangea blossoms can turn pink or blue depending on the soil pH. Pink blossoms emerge when the hydrangea is planted in alkaline soil; blue when it's in acid soil. If your hydrangea's blossoms are pale green, your soil has a fairly neutral pH. This makes it easier to turn blossoms blue than if the soil were highly alkaline and your flowers were pink. Changing flower color takes one full year (or sometimes two) and there is no way to rush the process without harming the plant.
Mix 1 tbsp. of aluminum sulfate in 1 gallon of water. Store this mixture in a plastic milk jug or other container.
Water your hydrangea until the soil becomes wet but not saturated. Then apply some of the aluminum sulfate mixture to the plant. Adding too much can harm your hydrangea by burning the roots. Attempt to spread the application of the gallon out over four to five waterings during the spring and summer.
Continue to apply the aluminum sulfate solution to your hydrangea until you have used up the entire gallon. Then wait to see how your hydrangea blooms the next year. If you've sufficiently lowered the soil pH enough, the new blooms will be blue-tinted.
Repeat this process in full the next growing season to increase the amount of blue in your hydrangea. If the blooms haven't changed from pale green to blue, the soil pH is still too high. But a second year of aluminum sulfate should prompt color change.