Here's some interesting info on the color of Hydrangeas...
The main influence that affects flower hydrangea color is soil pH, which is a measure of the soil’s acidity. The pH scale is from 0 to 14. Seven is the midpoint and is neutral. PH levels above 7 indicate alkalinity, while those below 7 represent increasing degrees of acidity. The average garden soil is slightly acidic in the range from 5.5 to 6.5. The pH range of 4.5 to 7.5 is not uncommon in garden soils. Blue hydrangeas require a highly acid soil, while pink hydrangeas are produced in a neutral or slightly acid soil.
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