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Colo(u)rs

Colours are visual sensations experienced by human brains when light enters the eyes.


PHYSICS

Visible light is a category of electromagnetic radiation — waves of photons travelling through space — containing wavelengths roughly between 400 and 700 nanometers (the visible spectrum).

Other named categories of electromagnetic radiation, in order from shortest to longest wavelengths (fastest to slowest frequencies) are: gamma rays, X-rays, ultraviolet, infrared, microwaves, and radio waves. The visible spectrum is between ultraviolet and infrared.


Photograph of the visible spectrum created from sunlight through a prism on paper

Individual wavelengths of light appear to us as different colours. We call these spectral colours.

More colours are witnessed when multiple wavelengths of light are mixed or blended together.


BIOLOGY

Four types of photoreceptor cells in human eyes help convert light into mental images.

'Rods' are the most sensitive when light is scarce, generating grey-scale signals to distinguish black darkness (the absence of light) from lit areas, appearing whiter or brighter as light grows more plentiful.

Three varieties of 'cones' work together to create all other colours — S-cones, M-cones, and L-cones — named for their relative sensitivities to short, medium, and long wavelengths of light.


Chart of the estimated relative response signal strength of cone cell types to varying wavelengths of light

Different colours are perceived by the brain when different mixtures of light frequencies result in different stimulation or excitement profiles across the three cone cell types.

With no light, we see black.

When all three cone types are stimulated equally, we see white (or grey, if the level of stimulation is not very strong).

By choosing three particular wavelength compositions of light, each fitting in a narrow band and primarily stimulating one of the three cone types, mixing them in different quantities can approximately replicate any feasible cone excitement profile and therefore any colour. This is the conceptual basis for the theory of 'primary' colours.


ARITHMETIC

The display screens of many electronic devices use an RGB system (red–green–blue) to generate all of their images. Red, green, and blue are commonly known as the additive primary colours, or the primary colours of light.

While no three realizable coloured lights can be blended to perfectly match every possible colour, the gamut covered by the standard primaries is wide enough such that it's tough to tell anything's missing.

Overlapping red, green, and blue light circles showing basic mixtures

So far, this discussion has focused on colours of light. It is also familiar to attribute colours to non-luminant objects.

Viewing an object like a flower or a painting, our eyes see any light reflected off its surface. Different materials chiefly reflect certain wavelengths and absorb others.

A black object absorbs all light so that none bounces into our eyes.

White objects reflect a broad frequency range to excite all of our cones.

Mixing coloured paints, inks, or dyes will yield a new compound whose wavelength-absorption profile is a combination of its constituents'. For example, a cyan ink (reflecting light blue) absorbs red light while a yellow ink absorbs blue light: a blend of cyan and yellow inks absorbs both red and blue lights, reflecting green. Adding to this result a third, magenta ink, which absorbs green, should give a dark or dull neutral colour which doesn't fully reflect any portion of the visible spectrum. This is called subtractive colour mixing, in contrast to the additive mixing of lights, and the commonly known subtractive primary colours are cyan, magenta, and yellow.


ART

Painters can produce varied and vibrant works using only red, blue, and yellow pigments (often they include white and black paints too, to adjust 'values' — the lightness or darkness of colours). Red, blue, and yellow are commonly considered primary colours, in subtractive mediums.

Adding more colours to your standard palette, however, can only help: a wider arsenal of mixable variations should assist in expressing your feelings and ideas.


An oil painting loosely following a colour wheel pattern

Studying a traditional 'colour wheel' is worthwhile for painters to train an instinct for mixing. All paints behave differently, though; there is no purest or perfect palette and there are no universal recipes.

Follow your heart, for art.








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