The color of your hydrangea is more about its soil than the variety chosen. Interestingly, you may purchase a pink-bloomed hydrangea plant. After transplanting, the blooms may change the next year to blue or even a combination of colors. It could be that the plant is readjusting to it's new environment. It could also mean that your ground soil has a different pH (acidity value), the wrong fertilizer or wrong soil nutrients. You will have more control over your hydrangea's bloom color if you plant them in containers. The change in ground soil can come and go as it's fertilized or water washes nutrients away.
Choose a pink or blue hydrangea. A pink hydrangea may be changed into a blue one. However, a blue hydrangea plant, if you can find one, is more likely to stay blue if it keeps it's ideal soil makeup.
Test your ground soil. pH testers can be found at your local nursery or garden center. The soil needs to be a low number, between 5.2 and 5.5. If your soil is pH value is too high, add compost (or other organic material). Also, add a bit of aluminum sulfate. Measure out ½ oz. and mix it in a gallon of water. Water the hydrangea plants well, after planting, and then pour the sulfate mixture into the soil around them.
Choose a planting location that gets plenty of morning sun and afternoon shade. Keep in mind the plants maturity size, which can reach 4 feet across and 4 feet in depth.
Dig a whole as deep as the purchased hydrangea plant container, but a bit wider. If your soil acidity is at the proper level, you may still want to add aluminum sulfate. With this soil configuration, The hydrangea plant blooms should turn blue with the next blooming season.
Remove the plant from it's original container, loosen the roots and place it in the hole. Push soil into the sides of the hole until the hydrangea is compactly structured.
Fertilize your hydrangea plant once or twice a year, during the summer months, with a high potassium, low phosphorus fertilizer. Avoid fertilizing late in the blooming season, which can go into fall with some varieties. If it has too much phosphorus, it will be difficult to get your blooms to turn blue. A good mixture would be a 25-5-30. This number on the fertilizer package means it has 25 parts nitrogen. 5 parts phosphorus, and 30 parts potassium