First of all, yes, hydrangeas grow well in Hawaii's cooler areas. I happen to have one bush on my homestead farm, though it's taken a beating getting stepped on by the livestock. But the buggah is one tough plant and still blooms every year. Like Fineartgourd's flowers, mine are turning blue.
Why bother to bring this topic up? Because hydrangeas are a natural soil pH indicator, assuming one very important condition......the presence of aluminum ions. Obviously Hawaiian soil has enough aluminum.
Acidic soil is not present everywhere on this island. In fact there are areas where the soil is far too alkaline. But I happen to live downwind from an erupting volcano. Thus our rain is very acidic. Combine that with a low calcium content of the soil, the soil becomes acidic enough to turn our hydrangea flowers blue. Up closer to the eruption site, in the village of Volcano, the flowers are even more blue.
It's just kind of fun to have Mother Nature's pH indicator growing on the farm.