Decorating: Flower Arrangement

It's summertime, and my hydrangea plant is in full bloom in our garden. I didn't want it to go to waste, so I decided to create a flower arrangement for our living room. When I planted the flowers a couple of years ago, they were all the same pink color. But now the ones in our backyard is purple, while the front yard has pink flowers. It has something to do with the soil, I guess.