In the case of the blue color two main strategies have been used by plants and studied in detail. They involve acylation of the sugars to allow intramolecular copigmentation and complexation with metals

Of all of these pigments, the only one capable of producing blue are the anthocyanins.

“blue coloration by intermolecular co-pigmentation alone is not common, and when it occurs, it is usually the result of co-pigmentation of anthocyanins with glycosylated flavonols and flavones”

Michel Pastoureau, “ Contrary to what one might imagine, the social, artistic and religious uses of the color blue do not reach back into the mists of time .”

The precursors of indigo can be extracted entirely from plants. The extract, a solution of the water-soluble indoxyl glucoside is hydrolyzed via fermentation to produce indoxyl, which will further react with another indoxyl molecule to produce indigo; or, in a side reaction, with isatin, resulting in indirubin, a reddish dye [15].

In conclusion, the blue color only appeared many million years after the first plants color systems and only through the anthocyanins, Fig. 3.

Blue was the last color to diffuse in Nature and the last to appear in many of the Humans’ discoveries.

Plants had to develop strategies to fix this color, mainly by intramolecular copigmentation in acylated anthocyanins and complexation with metals, as in metalloanthocyanins.

While less studied, structural color has also been reported in plants [7,10,12]. In plants, these optical effects arise from the multilayer interference of two or more materials with different refractive indexes, or by stacking layers of cellulose microfibrils with differing orientations forming a helicoid structure similar to a liquid crystal... See more