An Introduction to Colour

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Additive & Subtractive colours

Primary coloursRGB colour is all around us in the form of light, and is referred to as the Additive colour system. While far from perfect our colour monitors and television sets can reproduce up to 16.77 million RGB colours. If appropriate Red, Green and Blue filters are placed in front of high powered spot lights and the three lights directed to the same surface, and overlapping as shown in the image on the left, a white area would appear where ALL three lights overlap.

Where each light overlaps another we would see Cyan, Magenta or Yellow - the Subtractive colour system components.

If there is a total lack of light then we will see black or nothing. So total darkness would be described in RGB values as R=0,G=0 and B=0, and all visible light would be described as R=255, G=255 and B=255. Any value of Red Green or Blue is always between 0 and 255 which allows for a maximum of 256 values because to a computer, 0 is indeed a value whereas 0 means nothing in human terms. (therefore 256 x 256 x 256 = 16.77 million RGB colours)

Secondary coloursWe cannot print onto a surface with light. A printing press requires pigments of the Subtractive colour system (ink) in an attempt to match all colours of nature. However, not only are the CMY pigments incapable of matching all colours, their imperfections allow the reproduction of far less colour than our RGB devices. This raises many problems for the graphic reproducer. The image on the left shows a similar overlap of inks to the spotlights described above, and where an ink spot overlaps another spot we would hope to see Red, Green or Blue. Process inks have to be translucent yet retain the ability to achieve set standards of density. Using densitometry we can determine the amount of absorption (Light) and Reflectance (Light) i.e. ideally only the colours we want to see are reflected back to our eyes.
Where ALL three spots overlap we would hypothetically produce black but the inability of man to produce perfect pigments results in a muddy or dark brown.

Because CMY cannot produce Black we must add a fourth pigment to assist many colours reproduced on a press and make the application of 100% of CMY appear black. This fourth colour is indeed Black - often referred to as the KEY colour, hence the CMYK printing process.

CMY and K values can be 0% for none (white) to 100% for full colour (percentage halftone dot size).

Colour GamutsColour Gamut
The image on the left comparing colour gamuts represents firstly the number of colours of light that the average human eye can detect. The second or medium sized area represents comparatively the number of colours that can be reproduced by man with RGB devices. The third or smallest area represents the colours that can be reproduced using CMY pigments. While the coloured background is by no means an accurate representation of colours covered, the areas of each gamut differ markedly and this tells us that we face problems when images are produced by artist/designers who do not understand or appreciate these differences (i.e. a chosen RGB colour may not be printable). The problems amount to many millions of dollars wasted around the world each year and must not be treated lightly.
Correct colour choices will be looked at later and include a download sample for PhotoShop users.
Colour WheelThe Colour Wheel
Artists and Graphic Reproducers should understand the Colour Wheel that graphically describes how the Primary Subtractive colours fit in with the Primary Additive colour components. In between Red and Green we find Yellow, between Green and Blue we find Cyan and between Blue and Red we find Magenta. Also opposing Red is Cyan - Blue is Yellow - Green is Magenta.

Notice that I have described the colour associations in an anti-clockwise direction. Colour Wheels are always referred to in this manner and the first reference is always Red. Therefore Red = the 0 or 360 degrees angle position, Green = 120 degrees and Blue = 240 degrees. Another colour reference system that is offered in most software packages is the HSB colour reference (Hue, Saturation and Brightness) and is a 3 dimensional description. Hue is described as an angle i.e. Yellow = 120 degrees. Saturation is described as a percentage where the centre of the colour wheel is 0% and the outer perimeter is 100% at any angle (Hue). Brightness is the third dimension - not shown - and is a percentage White to Black.

Colours can be described from/to anywhere in the world by using one of several colour reference systems:
  • RGB i.e. 135,243,39 (values between 0 and 255).
  • HSB i.e. 289,100,50 (values = degrees,%,%).
  • CMYK i.e. 35,70,95,8 (values between 0% and 100% halftone dot size).
  • Other methods often used by scientists to convey imagery to any computer system.
  • Proprietary colour matching systems i.e. Pantone, Toyo Ink etc.. (Spot Colour or Process Colour?)

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