Changing color in a Hydrangea can be because of many factors. However, the most common cause is absence or presence of Aluminium in the soil. Hydrangeas are turning blue because there is more aluminium in the soil. In order to retain their white color, you would have to raise the PH of the soil. Add Dolomitic lime. The PH of the soil should be around 6.
Test your soil for acidity. Hydrangeas go blue in acid soils .Either move the scrub or feed with an alkaline rich fertilizer. This should even out the problem.