Hydrangeas that bloom with a good pink color are surely growing in a soil with a pH range between 6.5 and 7.0. If, however, your hydrangea plant has blossoms with variable colors ranging from pink to blue or perhaps even purplish pink, the soil pH is probably in the range of 6.0 to 6.5 and is considered to be slightly acid. A soil pH of 5.5 to 6.0, considered to be in the acid range, is required to produce a good clean blue. It is too late now to change the color of existing flowers, but it is an ideal time to begin working on the pH of the soil for next year's crop. Changing the pH of the soil takes several weeks, and this pH change must be accomplished prior to bloom bud initiation later this summer.
Some people prefer hydrangeas with a clean pink color.