(Chemical substances related to the colors of flowers: Flavonoids, carotenoids and chlorophyll)
(Reproduction authorized by Professor Doutor Agnaldo Arroio, from the Faculdade de Educação - USP
Departamento de Metodologia do Ensino e Educação Comparada. )
Revista Eletrônica de Ciências (http://www.cdcc.usp.br/ciencia/artigos/art_29/apredendo.html)

The colors of the leaves and the flowers of plants are determined by substances (pigments) present in the chemical composition that absorb certain bands of visible light and reflect the rest. The color we see is the reflected light which presents a complementary coloration to that absorbed by the plant.
Many colors we see in the plants depend on the presence, on leaves and petals of flowers, of molecules of substances called pigments (in some case, the tissue structure of the petals causes a blue color spreading, the same phenomenon that gives the color to the sky). The change of the leaves color of many species in autumn happens exactly because of those pigments alterations.
The pollinators usually show preference for some colors, not only the colors of the visible spectrum (the colors of the rainbow) but also the regions where we can't see, for example the ultraviolet. Bees have tendency to be attracted by yellow and blue colors. They are also able to perceive the ultraviolet region of the spectrum that is invisible to us. They are very sensitive to flavones and flavenoids which absorb ultraviolet and are present in almost every white flower. Bees are insensitive to red but they visit red flowers guided by the presence of flavones which absorb ultraviolet light.
The hummingbirds are only sensitive to red, and their preference for bright red flowers such as Hibiscus, is known. In particular habitats can also visit white flowers.
The other classes of pollinators show a lower sensitivity to the color of the flowers. While the butterflies are attracted to flowers of vibrant color, the moths prefer red flowers, purple, white or light pink and wasps prefer dull colors, dark and drab. Flies are attracted to flowers of dark, brown, purple or green. The beetles and bats, which are visually inert to the color, depend on another type of signs to be guided to the flowers especially the smell.
The characteristics of the smell of the plant are essential. Especially when we think about the pollination that occurs at night when the pollinators are attracted by the scent, since the visual appeal of the colors of the petals become more evident during the day. That is why we feel more smells of "nocturnal flowers', their aromas are stronger. The bees respond strongly to the stimulus of the aroma and have a preference for the smell we know as perfume. Due to high sensitivity of insects to the smell, even the flowers that seem not odor to the human smell, they contain sufficient quantities of material attractive.
There are three major groups of chemicals associated to the colors of flowers: the flavonoids, carotenoids and chlorophyll.
The flavonoids are a class of components produced by plants with high potential for bio activity and used in different ways. Anthocyanins are the pigments responsible for red, pink, purple and blue flavonoids. For humans, these substances have potential as bio active substances, and their role in nature is to attract insects for dispersal of seeds and pollen. Other flavonoids can absorb light at wavelengths shorter than those of anthocyanins and thus can not be seen by the human eye. However, bees and other insects can see ultraviolet and be attracted.
The anthocyanins are responsible for most of the red, purple and blue that we see in flowers. The three main pigments this subclass are pelargonidin (reflecting red light), cyanidin (which reflects light carmine) and delphinidin (which reflects light bluish-purple). The acidity of the medium in which they are anthocyanins can also influence the color they reflect.
Carotenoids include a family of natural compounds, of which more than 600 structural variants are reported and characterized, from bacteria, algae, fungi and higher plants. Carotenoids are pigments widely distributed in nature, responsible for the colors orange, yellow and red fruit, vegetables, flowers, birds and some fish, bacteria, algae, fungi and yeasts.
Carotenoids are also found in the leaves of many plants, but their presence is masked by the large amount of another pigment, chlorophyll. The molecule of chlorophyll is the pigment that absorb light in the bands of crimson and purple, reflects the green color that we see in leaves. The energy absorbed by chlorophyll through light is used in photosynthesis, where carbon dioxide and water combine to form carbohydrates, which are structural and nutritional roles in plants.
This enormous diversity of pigments, blended with the combinations generated by the simultaneous presence of different pigments in different plant parts, explains the infinite variety of colors found in nature.