Although not all hydrangea flowers change color, colors may differ seasonally, depending on the weather, stress on the plant and environmental changes, according to About.com. Different varieties of hydrangeas vary in color, size and blooming time. It is possible for the hydrangea's colors to change between pink and blue, depending on the soil's aluminum levels and pH balance. Altering and maintaining these colors is a common technique among growers.
According to About.com, hydrangea flowers turn blue when the soil is slightly acidic and when there is aluminum present in the soil.This is accomplished by using naturally acidic soil and adding aluminum sulfate. Gardeners use fertilizer that is high in potassium and low in phosphorous to maintain the blue color.
Hydrangeas turn pink when they have diminished aluminum levels. This also involves adding dolomitic lime to raise the soil pH. About.com also recommends growing pink hydrangeas in pots and using soilless potting mixes, which usually lack aluminum. Phosphorous prevents hydrangeas from accessing aluminum, so About.com recommends using a fertilizer rich in phosphorous to maintain a pink hue.Learn more about Outdoor Plants & Flowers