In most species the flowers are white, but in some species (notably H. macrophylla), can be blue, red, pink, light purple, or dark purple. In these species the color is affected by the pH of the soil; acidic soils produce blue flowers, neutral soils produce very pale cream petals, and alkaline soils results in pink or purple. This is caused by a color change of the flower pigments in the presence of aluminium ions which can be taken up into hyperaccumulating plants.
To enhance beautiful blooms for next year remove all stems that support faded flowers on your blue and pink hydrangeas; shorten droopy, flowerless stems by one-third. The new growth that occurs between now and winter will produce next summer’s blooms. See:
*The Georgia Gardener online