Re: how do flowers get their color?

Flower colors of red, pink, blue and purple come mainly from the pigments 
called anthocyanins. Anthocyanins are water soluble pigments in the class of 
chemicals called flavanoids, which are phenolics. Anthocyanins are found in the 
cell vacuoles and are glucosides meaning they contain a sugar molecule. 

A classic experiment uses the anthocyanins in red cabbage as a pH indicator 
because anthocyanins change color depending on the pH. You could do the same 
thing with flower petals. An easy way is to crush a red rose petal on a white 
plate with the back of a spoon and add either vinegar (acid) or baking soda 
(base) and note the color change. A college lab manual has an experiment on 
chromatography of flower anthocyanins (Witham et al. 1971).

Flowers may also contain another class of pigments called carotenoids, which 
are responsible for red in tomato and orange in carrot root. Carotenoids are 
red, yellow and orange in color. They do not reside in the vacuole but in 
organelles called plastids, either in green chloroplasts or in nongreen 
chromoplasts.

References


Red Cabbage pH Indicator


Red/Purple Cabbage pH Indicator

Genetics and Biochemistry of Anthocyanin Biosynthesis The Plant Cell
Vol 7, 1071-1083, 1995. (PDF file).

Witham, F.H. et al. 1971. Experiments in Plant Physiology. New York: Van 
Nostrand Reinhold.