Colour models

There are three colour models used in graphic art. RGB, CMYK and HSB

RGB refers to the three light primaries RED, GREEN and BLUE, used to produce the colour in projections and computer screens.

CMYK refers to four inks used in some printing, CYAN, MAGENTA, YELLOW, and KEY (BLACK). To save money on ink, and to produce deeper black tones, unsaturated and dark colours are produced by using black ink instead of the combination of cyan, magenta and yellow.

HSB refers to HUE, SATURATION and BRIGHTNESS.

  • Hue refers to the position of a colour on the standard colour wheel in degrees from 0o to 360o.
  • Saturation refers to the strength of the colour. Saturation (or chroma) represents the amount of grey in proportion to the hue, measured as a percentage from 0% (gray) to 100% (fully saturated).
  • Brightness is relative lightness or darkness of the colour measured from 0% (black) to 100% (white).
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CMYK is a subtractive colour model used in printing while RGB is the additive colour system. Subtractive colour means that the colours mix like paint, while additive colour means they mix like light.

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Additive colour mixing

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Subtractive colour mixing

A colour space is a particular implementation of a colour model. Within each colour model there are a number of colour spaces, for instance within the RGB colour model there are AdobeĀ® RGB, sRGB, and AppleĀ® RGB colour spaces. A colour space represents its own gamut (range) of colours. Each device can only produce colours in its own space, and since these spaces are not the same, so colours change as files are moved from one device to another. This is why printed copies of an image do not perfectly match the colours as seen on the screen. The RGB colour space of the monitor and the CMYK colour space of the printer have different gamuts, so some colours produced by the printer cannot be reproduced on the screen and some colours in the screen cannot be reproduced by the printer.

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