The color of the typical mophead hydrangeas depends on the amount of aluminum available to them. If the soil is deficient in aluminum then the blooms will be pink or red. If the soil has plenty of aluminum but is alkaline then the aluminum is tied up in the soil and is unavailable to the plant. The blooms in this case will be pink or red. If the soil has plenty of aluminum and the soil pH is acidic then the blooms will be blue. The blooms will be purple if there is a low concentration of free aluminum in the soil. This occurs from a neutral pH or a low concentration of aluminum in the soil.

In your picture, some parts of the plant are getting enough aluminum (blue), some part aren't (red) and some parts are in between (purple). This is also common with newly planted hydrangeas for the first couple of years. while the root system is getting established.