When physics and chemistry combine for non-spectral colors.

The pigment/photonic structure combination to generate complex colors is usual in nature. The most common case is green, which can be created by a green pigment, such as chlorophyll in plants, or a combination of blue and yellow. In animals, and insects in particular, it is often this last strategy that is chosen. It is an economical solution, blue and yellow being easily created by photonic structures for the first, and by a pigment - generally a melanin - for the second. Strictly pigmentary green, on the contrary, requires complex molecules capable of absorbing at both ends of the visible spectrum, and reflecting the center.

Purples are by definition this complex color type. They are created by mixing violet and red and are called non-spectral as no wavelength can be associated with them. They are assigned the wavelength of their complementary, green, preceded by the sign -. The color of the purple spots on the ventral side of Lyropteryx apollonia butterfly is thus generated by the combination of a red pigment forming a uniform background on the very convex cover scales, and a violet created by a photonic structure of the Morpho type: a stacking of lamella in each striae of the scale, only visible to our eyes on the crest of the scales. This mixture of violet lines on a uniform red background generates an iridescent purple, the structural violet depending on the incidence of light. A clever combination to create a rare color, a privilege of Roman emperors and a communication channel for a modest butterfly.

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