RGB Colour Wheel with 18 Colours – Wheel

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This is one of a series of diagrams exploring RGB colour wheels. Colour wheels demonstrate or simulate the affect of colour mixing.

Colour wheels can be used to explore the effect of mixing any type of colour. Light, inks, dyes, artist’s paints, pigments and colourants all produce other colours when mixed together.

Whilst a colour model outlines a method for mixing and using different types of colour, a colour wheel explores what happens in practice.

Understanding the diagram

• A colour wheel fills the centre of the diagram and in this case has three segments showing the three RGB primary colours with five intermediate colours between each one.
• The RGB codes corresponding with each colour appear to the left of the wheel and continue on the right.
• Notice how the colour notation works:
  • Three numbers (separated by commas) show how much red, green and blue light is used to produce each colour.
  • The minimum value for each light source is 0. In this case the light is fully off.
  • The maximum value for each light source is 255. In this case the light source is fully on.
  • As each number increases so does the intensity of the corresponding light but the wavelength and so the colour we see stays the same.

What is an RGB colour wheel

• The purpose of an RGB colour wheel is to demonstrate or simulate the effect of projecting coloured lights, corresponding with the additive primaries (red, green and blue), onto a dark surface.
• In laboratory conditions, light sources are of equal intensity when fully on but can be turned down in 256 equal steps till fully off. The wavelengths of the light they output are set to values such as:
  • • Red = 625 nanometres
    • Green = 500 nanometres
    • Blue = 440 nanometres
• When coloured lights are projected and focused onto a surface they form overlapping circles. Colour wheels however are usually divided into segments like the spokes of a wheel on a bicycle.
• For everyday purposes, the most straight forward way to explore the RGB colour model is on a computer using software that allows different RGB colours to be selected.
• RGB colour wheels have a minimum of three segments. These are filled with the red, green and blue additive primary colours.
• When exploring RGB colour wheels the first thing to establish is what happens where pairs of primary colours of equal intensities overlap.
  • • Where red and green light sources overlap they produce yellow.
    • Where green and blue light sources overlap they produce cyan.
    • Where blue and a red light sources overlap they produce magenta.
• Yellow, cyan and magenta are called secondary colours and fill the segments between the primary colours on a colour wheel with six segments.
• Mixtures of equal intensities of pairs of secondary colours are called tertiary colours on a colour wheel with twelve segments.
• Additional colours are produced by continuing to overlap equal intensities of adjacent pairs of colours.
• The range of colours that can be produced on a computer screen by an RGB colour wheel is limited only by the system of notation, the resolution of the device they are displayed on and by the ability of an observer to distinguish between similar colours.

About RGB colours

RGB colours are produced:

• On a computer or mobile phone screen by juxtaposing tiny dots of light corresponding with the three primary colours, red, green and blue.
• In computer software and apps by selecting RGB colours using swatches or by selecting RGB colour values (codes) using decimal or hexadecimal notation.

Look at a computer screen or TV using a magnifying glass to see the three RGB primary colours. Then step back to see how different colours appear when all the pixels merge together.

RGB and spectral colour

• The RGB colour model (RGB colour) should not be confused with the spectral colour model (spectral colour). Spectral colours relate to the visible spectrum and so to the rainbow colours ROYGBV.
• The human eye is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the human perception of colour.
• Rainbow colours are the bands of colour seen in rainbows and in other situations where visible light separates into its component wavelengths.
• It is an artefact of human vision that spectral colours corresponding with all the wavelengths of the visible spectrum appear as bands of colour to a human observer.

RGB colour values

RGB colour values are represented by decimal triplets (base 10) or hexadecimal triplets (base 16). These are used in software and apps to select a colour.

In decimal notation, an RGB triplet is used to represent the values of red, then green, then blue.

A range of decimal numbers from 0 to 255 can be selected for each value:

• Red = 255, 00, 00
• Yellow = 255, 255, 0
• Green = 00, 255, 00
• Cyan = 00, 255, 255
• Blue = 00, 00, 255
• Magenta = 255, 00, 255

In hexadecimal notation an RGB triplet is used to represent the value of red, then green, then blue. A range of hexadecimal numbers from 00 to FF can be selected for each value.

The hash symbol (#) is used to indicate hex notation:

• Red = #FF0000
• Yellow = #FFFF00
• Green = #00FF00
• Cyan = 00FFFF
• Blue = #0000FF
• Magenta = #FF00FF

The sequence of hexadecimal values between 1 and 16 = 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E and F.

The sequence of hexadecimal values between 17 and 32 = 10,11,12,13,14,15,16,17,18,19,1A,1B,1C,1D,1E and 1F.

About the RGB colour model

• RGB colour is an additive colour model that combines wavelengths of light corresponding with the red, green and blue primary colours to produce other colours.
• RGB colour is called a model because it is a method that can be followed to produce any colour from a combination of red, green and blue light sources.
• Red, green and blue are called additive primary colours in an RGB colour model because they can be added together to produce other colours.
• When mixing light, each RGB primary colour is called a component of the resulting colour.
• Different colours are produced by varying the intensity of the component colours between fully off and fully on.
• When the light sources that produce the red, green and blue primary colours are at full intensity, together they produce white.
• Each light source at full intensity produces a fully saturated colour.
• When any two fully saturated RGB primaries are combined they produce a secondary colour (yellow, cyan or magenta).
• Some applications of the RGB colour model can produce over 16 million colours by varying the intensity of each of the three component primary colours.
• The additive RGB colour model cannot be used for mixing opaque pigments, paints or powders. To understand these colourants find out about subtractive colour models.
• The RGB colour model does not define precise wavelengths (or band of wavelengths) for the three primary colours.
• When the exact composition of primary colours are defined, the colour model then describes an absolute colour space.