The color of a hydrangea bloom depends on the chemical makeup of the soil it is planted in. If the soil is high in aluminum and has a low pH, the hydrangea flower will be blue. If the soil has either a high pH or is low on aluminum, the hydrangea flower color will be pink. In order to make a hydrangea change color, you have to change the chemical composition of the soil it grows in.
My blooms started out purple for the first few years. Now they are changing over to blue. Amazing what nature offers all of us. You can still see a tinge of purple in some of the blooms. I prefer the blue. We all see what my spring garden yields this year.
Have you had a flower change colors in your garden?