For an alternative approach to answering the question of what are
the primary colours perhaps we should first answer the question of what makes a colour �primary�.
While adding light of specific wavelengths centered around red blue and green do approximate the perception of other colours such as yellow orange cyan megenta these perceptions are an illusion brought about by virtue of the process our eyes use to discriminate colours. Colour discrimination is achieved visually by comparing intensities of three bands of colour centered around the frequencies of red blue and green to which three different types of cones in the retina of the eye are most sensitive. Mixing various colours of light does not produce a new intermediate wavelength. The eye 'sees' the true wavelength of, for instance, "yellow" by the affect that wavelength of light has of stimulating in certain ratios the red, blue and green receptors of the retina.
The ability to stimulate the full gamit of colours that the eye can distinguish is best accomplished by choosing the three wavelengths of light that produce the most discrete stimulation to each of the three types of cones in the eye. Although the three most suitable colours for this purpose vary slightly between individuals, testing a large sample of people with �normal� visual responses allows us to choose the three specific wavelengths of light which produce the greatest possible gamit of colour variation and range among the largest range of viewers. These colours of red, green, and blue would then be defined by the specific wavelength of each.
In practice a certain amount of compromise is taken in selecting the actual colours used due to availability and efficiency in producing the ideal wavelengths.
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