Liz, with the macrophylla hydrangeas, it is possible to see varying hues of pink, turning purple to blue, all on one plant. Sometimes one inflouresence will exhibit several shades of color. The root system is deep and wide on a mature plant and depending upon where those roots are feeding, different parts of the plant may be feeding from a more acid or alkaline soil!

Lut posted a pic of a Hydrangea aborescens, cv. "Annabelle. I think that's the "white one" you're thinking of!
Lut's Hydrangea