What are RGB and CMYK in Photography and Photoshop?

Published: 17th April 2009
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Light is made up of red, green and blue; but there is also cyan, magenta and yellow. These two sets of colours are colour modes, so how do they fit together and what are there relevance in Photography and in learning Photoshop?

It is possible to change and adjust the colours of a digital image as much as we please, but to do so it is necessary to understand something about colours and how they combine and work together.
The basic colour modes are Red, Green, Blue (RGB) and Cyan, Magenta, Yellow, Black (CMYK). They are known as colour spaces and they determine how images are seen and reproduced.

Electromagnetic radiation comes in the form of wavelengths and its total range is known as the spectrum. The range covers gamma rays, X-rays, ultra-violet, visible light, infra-red, radar, microwaves, various radio waves and extremely low frequencies.

The only part of this spectrum which can be seen by the human eye is a very small section in the middle seen as white light. White light is refracted into its component colours of red, orange, yellow, green, blue, indigo and violet, which we see as the rainbow colours. The primary colours are red, green and blue.

If these three colours are projected individually on to a white background, where they overlap in the middle they blend to produce white light.

Objects reflect or transmit certain wavelengths and absorb others. It's the way the eye and brain react to these wavelengths that gives an object its colour. No object actually has any colour. What we see as colour depends on the wavelengths a particular object reflects and how the human eye and brain react to them. Often animals cannot see colours the way we can; that is because their brains are not designed to interpret those wavelengths.

So red, green and blue make up white light, but the proportions of colours within white light vary and are measured on the colour temperature scale. This scale comes from heating up a black object; it's colour will change the hotter it becomes. Imagine heating up the element on an electric cooker. It starts as cold, black metal; as it gets hotter, it starts to turn dark red, then lighter red and eventually becomes white hot. The numbers on the scale represent the temperatures as the colour changes.

Photographic daylight film is set to a colour temperature of 5500K on the scale and is a good balance of the component colours. Tungsten light contains much more red light than blue. At sunset, the sun's rays have to pass obliquely through more atmosphere which filters out blue light so we see the sunset as red. Go up into the mountains and in the snow there is more ultra-violet light so the white light has a strong blue bias. Water filters out red light so underwater objects start to look more blue also.

The human eye and brain can automatically white-balance a scene and the brain does a good job smoothing out such discrepancies. Because of this, a piece of white paper will still look white in a vast range of shades of white light, but the colour content differences are still there.
Film, however, will notice quite subtle shifts in colour.
Camera electronic files will notice colour shifts even more pronouncedly than film.

Transparency or positive film uses the primary colours of Red, Green and Blue, also known as additive colours, and when mixed they can create millions of colours. Negative films use the opposite colours of Cyan, Magenta and Yellow, also referred to as secondary or subtractive colours. Even though RGB can produce such a vibrate range of colours, the problem is it does not give the best results when it comes to printing with coloured inks. Here it becomes necessary to use cyan, magenta and yellow inks.

We need to understand how all the colours and the two modes are connected.

When cyan, magenta and yellow are projected on to our white background, because they are subtractive colours, they take their own colour out of the white light. Take out all three colours and we are left with no light and hence black. This will be seen in the black triangle where cyan, magenta and yellow overlap. When printing on white paper, we are in effect taking out light.

  • Starting with white light, taking out green leaves red and blue; where the two overlap they make magenta - the opposite of green is magenta.

  • Starting with white light, taking out blue leaves red and green; where they overlap creates yellow - yellow is the opposite of blue.

  • Using only yellow and magenta creates red where they blend together - cyan is left out and red is the opposite of cyan.

The title of this section mentions CMYK; the K is for Black. The reason is that when cyan, magenta and yellow inks are mixed together, they create a black but it remains somewhat dirty looking. To use 100% each of CMY would be expensive on ink and would create drying problems. A 100K black ink combined with 50C or perhaps 50C, 50M and 25Y will give a richer black.

More on the subject can be found via Photoshop In A Day

Duncan Wherrett is an experienced professional photographer and Photoshop instructor.
More great information and tips on Photoshop can be found at: ---->
How to Use Photoshop

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