My brother lives in a neighbourhood in Vancouver BC that has the most incredible hydrangeas. The colour is determined by more than the acidity of the soil. The acidity or alkalinity can enhance the colour, but the plant itself determines the colour.
In the blocks around his house are dark blue, magenta, pink, pale blue, white, pink (every shade imaginable) dark purple and more. In general the soil in Vancouver is acidic.
The one pink blossom in your photo may be a late bloom that has come after the plant was fertilized.