Colour is the visual perceptual property corresponding to the categories called red, blue, yellow, etc. Colour derives from the spectrum of light interacting with the photoreceptors in the human eye and the processing of this information in the brain. Colour categories and physical specifications of colour are associated with objects or materials based on their physical properties such as light absorption, reflection, or emission spectra. By defining a colour space, colours can be identified numerically by their coordinates.
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 380 to 750 nm. In terms of frequency, this corresponds to a band in the vicinity of 400–790 THz. The spectrum does not contain all the colours that the human eyes and brain can distinguish. Unsaturated colours such as pink, or purple variations like magenta, are absent, for example, because they can only be made by a mix of multiple wavelengths. Colours containing only one wavelength are also called pure colours or spectral colours.
Visible wavelengths pass largely unattenuated through the Earth’s atmosphere via the “optical window” region of the electromagnetic spectrum. An example of this phenomenon is when clean air scatters blue light more than wavelengths toward the red, giving the mid-day sky a blue hue. This optical window is also referred to as the “visible window” because it overlaps the human visible response spectrum. The near infrared (NIR) window lies just out of the human vision, as well as the Medium Wavelength IR (MWIR) window and the Long Wavelength or Far Infrared (LWIR or FIR) window, although other animals may experience them.
Color spectrum
| Color | Wavelength (nm) | Frequency (THz) |
|---|---|---|
| Red | ~700 | ~430 |
| Orange | ~620 | ~480 |
| Yellow | ~580 | ~520 |
| Green | ~550 | ~540 |
| Blue | ~475 | ~630 |
| Violet | ~425 | ~710 |
Definitions of colour
There are a few different ways to think about and define colour:
Wavelength
As mentioned above, colour is determined by the wavelength of visible light. Each colour has a specific wavelength range associated with it. Red light has wavelengths around 700nm, orange is around 600nm, yellow is around 580nm, green is 500-550nm, blue is 475nm, and violet light has wavelengths around 400-450nm.
Frequency
Colour can also be defined by the frequency of the light wave. Light frequency is measured in terahertz (THz). Red light has frequencies around 430 THz, orange is 480 THz, yellow is 520 THz, green is 540 THz, blue is 620 THz and violet light has a frequency around 700 THz.
Hue
Hue refers to the dominant wavelength or colour (red, orange, etc.). It is what we typically think of as ‘colour’. Hue is defined by position along the visible spectrum.
Saturation
Saturation (also known as intensity or chroma) refers to how pure or vivid the colour is. Highly saturated colours are very intense, while less saturated colours are more muted or greyish.
Brightness
Brightness (or lightness/value) indicates how light or dark the colour is. It ranges from black (minimum brightness) to the brightest colour (maximum brightness).
RGB values
RGB (red, green, blue) is a colour model that represents colours in terms of their red, green, and blue components. RGB values are specified using number from 0-255 for each component. For example, red is (255, 0, 0), green is (0, 255, 0), blue is (0, 0, 255).
Hex code
Hex code represents colour using hexadecimal digits (base 16 notation). Red is #FF0000, green is #00FF00, and blue is #0000FF. Hex codes are a compact way to specify RGB values.
Properties of colour
Some key properties of colour include:
Hue
Hue refers to the dominant colour/wavelength (red, orange, yellow, green, blue, violet). It is what we typically think of as ‘colour’.
Saturation
Saturation (also called intensity or chroma) refers to the purity or vividness of a colour. High saturation means very intense, vivid colour. Low saturation is more grey, muted colour.
Brightness
Brightness indicates how light or dark a colour is. It ranges from black (minimum brightness) to the brightest, most intense version of the colour (maximum brightness).
Shade
Shade refers to a colour that has been mixed with black to make it darker. Adding black reduces the brightness.
Tint
Tint refers to a colour that has been mixed with white to make it lighter. Adding white increases the brightness.
Temperature
Colour temperature describes the relative warmth or coolness of white light. Warm colours like red, orange, and yellow have lower colour temperatures (~2500-5000K). Cool colours like blue and violet have higher temperatures (5000-10,000K).
How we see colour
Human colour vision relies on specialised photoreceptor cells called cones that are located in the retina at the back of the eye. There are three types of cones that are each sensitive to different wavelengths of light:
S cones (short wavelength)
S cones absorb short wavelength blue light. They are primarily responsible for our ability to see blue colours.
M cones (medium wavelength)
M cones absorb middle wavelength green light. They detect green and yellow-green colours.
L cones (long wavelength)
L cones absorb longer red and orange light. They are responsible for red colour vision.
The brain combines and processes signals from these three cone types to produce the perception of a full range of colours. Problems with any of the cone types can lead to colour vision deficiencies.
In addition to cones, rods cells in the retina allow for vision in low light conditions. However, rods do not detect colour.
Primary colours
There are two main sets of primary colours:
Additive primaries (RGB)
Red, green, and blue light are the additive primaries. Combining light of these colours produces other colours. For example, red + blue light makes magenta. This is the principle behind TV screens, computer monitors, and other colour displays that emit light.
Subtractive primaries (CMYK)
Cyan, magenta, yellow, and black are the subtractive primaries used in mixing pigments, dyes, inks, and paints. Combining these pigments absorbs some wavelengths of light and reflects others, producing different colours. For example, cyan and yellow make green.
Complementary colours
Complementary colours are pairs of colours located opposite each other on the colour wheel. Common complementary pairs include:
- Red & green
- Yellow & purple
- Blue & orange
When complementary colours are mixed together, they cancel each other out to produce grey or white.
Colour theory & harmony
Understanding colour theory helps artists, designers, and others create more harmonious and aesthetically pleasing colour combinations. Important principles include:
Colour wheel
The colour wheel shows relationships between colours. Complementary colours are located opposite each other. Analogous colours sit side-by-side. Triadic colours form a triangle.
Colour harmony
Harmonious colour combinations use colours located close together on the colour wheel. Analogous schemes use adjacent colours. Complementary schemes combine opposites.
Context
The surrounding colours influence how a colour is perceived. A red shape on a blue background will pop, while the same red on a red background blends in.
Cultural meanings
Colour symbolism and associations vary across cultures. White means purity in Western cultures but mourning in some Asian cultures. Red means good luck in China but danger in Western cultures.
Mixing colours
Colours can be mixed in a few different ways:
Additive mixing
Adding light together makes new colours. Red, green, and blue light combine to make other colours. Computer screens and TVs mix colour this way.
Subtractive mixing
Mixing pigments/dyes together absorbs some wavelengths and reflects others, making a new colour. Cyan, magenta, and yellow are common subtractive primaries.
Optical mixing
Blending coloured dots of paint so closely that they are not distinguished individually by the eye. Pointillism relies on optical mixing of pure colours.
Conclusion
In summary, colour comes from light interacting with our eyes and brain. It can be defined by wavelength, frequency, or RGB values. Important properties include hue, saturation, and brightness. Complementary colours are pairs located opposite each other on the colour wheel. Colour theory principles help create harmonious colour combinations. Colours can be mixed additively with light or subtractively with pigments.
