Hydrangea flowers color is determined by the PH of soil. In most species the flowers are white, but in some species (notably H. macrophylla), it can be blue, red, pink, light purple, or dark purple. In these species the color is affected by soil pH. For H. macrophylla and H. serrata cultivars, the flower color can be determined by
the relative acidity of the soil: an acidic soil (pH below 7), by the influence of sundry agents in the soil, such as alum or iron, will usually produce flower color closer to blue, whereas an alkaline soil will produce pinker flowers. 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.