The most probable reason is that the tendency toward a particular tissue color in plants is genetically predetermined, but cultural conditions often play a big role in determining what color tissues will appear. Color can be changed by temperature and both light quality and quantity as well as pH and presence/absence of various micro-nutrients and metals. This explains why plants may flower differently in different environments (Hydrangea is a prime example).

Another possible probable reason is the effect of changes in pH at the cellular level. As cellular pH increases, the pigment changes structure and reflects different wavelengths of light. For example, in Hydrangea, lowering the pH of soil can result in more aluminum being available for plant uptake. The increase in aluminum uptake raises cellular pH. The resultant effect on pigments causes a change in the wavelengths of light reflected toward the blue portion of the spectrum. We perceive it as a change in bloom color.

The three pigments - chlorophyll, flavonoids, and carotenoids - combine in various proportions to give color to plant tissues. By mixing and matching the three pigments, an endless array of colors are possible.

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