Why do some hydrangea bushes look red and others blue

Why do some hydrangea bushes look red and others blue  

The name hydrangea derives from classical Greek mythology, in which the hydra was a beast with many heads. 

The chromophore in hydrangeas is delphinidin (X), which is a member of the anthrocyanidin class of compounds. Compound X reminds us of phenol (VII), indicating that delphinidin is also a weak acid. In fact, all pH indicators are weak acids or weak bases, and the ability to change colour is a visible manifestation of the indicator s ability to undergo reversible changes in structure. In the laboratory, only a tiny amount of the pH indicator is added to the titration solution, so it is really just a probe of the solution pH. It does not participate in the acid-base reaction, except insofar as its own structure changes with the solution pH. 

As an example, whereas the anthracene-based core of molecular X is relatively inert, the side-chain X is remarkably sensitive to the pH of its surroundings (principally, to the pH of the solution in which it dissolves). 

The word chromophore comes from two Greek words, khro-mos meaning colour and phoro, which means to give or to impart. A chromophore is therefore a species imparting colour. 

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