Changing colour is among the simplest tasks for which to design nanos. Polychromatic irisis, hair and skin may all simulate perfectly any natural colour, and to change between these tints effectively instantaneously. As well as realistic pigmentation, the polychromatics may be used to produce wildly unnatural colour schemes. More sophisticated models enable the display of animated patterns. The somewhat chaotic nature of hair generally limits such displays to relatively simple subjects such as fires, clouds or rainbows. The smooth surfaces of skin or the eyes may be used to display more coherent patterns, including video feeds.
The colouring is usually provided by a layer of complex aromatic compounds bounded to a substrate and covered by a diamond pane. The substrate contains chemical mechanisms which slightly change the electronic energy levels of the aromatics, thus modifying the colour. The surface can easily be divided into an array of cells, or chromophores, each of which can be independently changed in colour under the control of an electronic or rod-logic network. The system is very energy efficient as energy is required to change the colour of a cell, but not to maintain the colouration.
Chromocells can easily be constructed within the skin or the surface of the eye (usually, of course, within the iris). Hair provides a greater challenge, but specialist limited-assemblers may be installed within each follicle to incorporate chromocells within each hair. The lack of both blood circulation and temperature gradients within the hair means that the colour-changing apparatus in the chromocells must be powered by collecting incident light. In any case, the power consumption is so low that their activity is not limited by energy supply.
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