In a large hydrangea bush with a large root structure the distribution of ph in the soil is going to be a little varied. Where the ph is higher, the blooms will be pinker, and will be blue where the ph is lower. If I remember correctly the element is aluminum that creates the variance in color. The higher the aluminum sulphate the pinker the bloom will be, and the opposite if blue. So that's why on a single plant there can be more than one color, including a purply tint; it all depends on the soil content where that branch's root is feeding from. Of course you can control it better if you keep the bush potted, but that could limit the size. Is that all clear as mud?? Hard to explain, you almost have to visualize it, I guess.
On the other hand, I could be totally wrong, but it seems that way to me.
If you have a bush with white blooms, they'll remain white throughout their life, but will sometimes get a pink tinge as the blooms age.
