We had these growing up, but we called them snowballs... ours were always just white... maybe there are different varieties...
In any event, Mary says:
I cut back the hydrangea along the side of my house last fall (the gardener did
most of the work) and they are healthier now. This year some are very pink -
usually those are all blue. It's all based on the pH level of the soil, although
i didn't deliberately change it. There are three separate plants here. Maybe all
the bunnies in the neighborhood did something to the soil.