"Three different pigment groups - chlorophyll, flavonoids, and carotenoids - mixed in different proportions, give color to plants. By mixing and matching the three pigments, a near endless variety of colors can be created. E.g. most reds are the result of mixing orange carotenoids with magenta flavonoids. Cellular pH (not to be confused with soil pH) can have a profound effect on plant color.

Even most experienced gardeners think that lowering soil pH produces blue blooms in hydrangea. Technically, it is only a part of the equation. Lowering soil pH makes aluminum ions more readily available for plant uptake. The aluminum is then available to bond with pigment compounds (anthocyanin, the colored component of flavonoids) changing the way color is reflected. Our perception is, blooms changing from pink to blue.

The anthocyanidin group is what makes apples, autumn leaves, roses, strawberries, and cranberries red. They make blueberries, cornflowers, and violets blue. They also make some grapes, blackberries, and red cabbage purple. One of the things that changes the color of anthocyanins
is the level of acid or alkali (the pH) in the cell surrounding the pigment. As cellular pH increases, the pigment changes structure and reflects different wavelengths of light. The anthocyanin reflects bright pink in acid cell environments, reddish-purple in neutral and green in more alkaline cells ...."

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