Notes on Colour

visible light

Colour perception

Colour begins with light—light and the human eye. Our eyes respond unevenly to the range of wavelengths in the visible spectrum, which we see as bands of colour.

Famously, Newton counted seven colours. How we divide the range into colours depends on our language and culture, but there are interesting consistencies across languages. The edges of the bands are fuzzy. The points of division between colours can vary considerably, even for one person, but the central core concept of red, for example, is remarkably consistent across languages.

We see colours because different types of light receptors in our eyes are sensitive to different wavelengths of light. These receptors are called cones. We have three types of cone. They respond, more or less, to the red, green and blue regions of the spectrum. The responses are complicated and overlapping, but we can arrive at an effective understanding of colour if we simply say that cones respond to three primary colours—red, green and blue—which we call the additive primaries.

We distinguish all colours by the levels of response of the three kinds of cone. For example, we see a bluish green when there is a strong response from the green cones, less response from the blue cones and only a weak response from the red cones. We see yellow when there is a roughly equal response from the red and green cones, and almost no response from the blue cones. You can get a rough idea how this works from the illustrations.

 an illustration of the visible spectrum

Rvery approximate illustration of response of range red cones

Gvery approximate illustration of response of range green cones

Bvery approximate illustration of response of range blue cones

Extraspectral colours

Both blue-green and yellow are spectral colours. We also see colours that don’t appear in the spectrum, the extraspectral colours. Mixtures of red and blue light give rise to purple and magenta hues. The full range of hues is conventionally depicted as a colour wheel. If all three kinds of cone respond equally, we see white (or shades of grey, depending on the overall intensity of the light). Daylight is white light because sunlight has a fairly even balance of wavelengths in the red, green and blue regions.