Interestingly, the color of hydrangeas can change depending upon the acidity of the soil. Most garden soils have enough aluminum for proper cultivation, but the aluminum may not be available to the plant if the soil PH level is high. For most bigleaf hydrangeas, blue flowers will be produced in acidic soil of pH 5.5 and lower. The natural soil or more alkaline soils of PH 6.5 and higher often produce pink flowers. PH levels between 5.5 and pH 6.5, will be purple or a mixture of blue and pink flowers on the same plant.