Certified GKH Gardening Expert
Your soil needs acid and aluminum. Here's an article on how to acidify your soil:
and how to add aluminum:
Purple colored mophead hydrangeas are usually a reflection of having the proper DNA and soil that is close to neutral...neither too acidic nor too alkaline. And it is generally cultivars that would produce a vivid blue in acidic soils with adequate aluminum that take on the deeper purple hues.
And some cultivars are more stable in their coloring that others....typically those that tend to be pinks: Pia/Pink Elf, Glowing Embers, Teller Red, etc. Unless planted in very acidic soils, these tend to stay on the pink end of the scale. Varieties that resist turning blue in acidic soils -say, Merritts Supreme, Mathilda Gutges, Glowing Embers- will turn purple first before turning pink. But purple is always a tough one because it occurs in a very narrow pH range. Some varieties like Bloomstruck are bred to produce multiple colors within the same plant and purples tend to occur. Plant wholesalers also tweak the soil diet to enhance the purples.
In my alkaline soil, I add some aluminum sulfate to an unnamed hydrangea lacecap (possibly Twist & Shout) such that it is slightly acidic and takes a purple-lavender tone. I was originally aiming for blues but liked this color better. ;o))