|
Fundamental Concepts: Colour Theory
Computers and digital cameras are able to create the vast range of colours by mixing different proportions of the three additive primary colours these are red, green and blue. If there are any artists reading this and thinking the authors of this book dont have a clue what they are talking about read the section below on Red-Blue-Yellow.
Red, Green and Blue (often abbreviated to RGB) are the additive primary colours of light the ones that your camera sees and that every computer monitor and TV throughout the world uses to display its images. Mixing all three additive primaries together gives you white. These three colours enable your camera to capture a vast array of colours (but by no means all colours).
Subtractive primaries are the colours used by most printers usually cyan, magenta and yellow (and abbreviated to CMY). These are called subtractive primaries as the pigments or dyes that make up the ink/paint absorb (or subtract) specific colours from the white ambient light which is illuminating them. For example the cyan ink will absorb red light but not the green and blue. If you mix all three subtractive primaries, then the ink is absorbing all three additive primaries (RGB) in the ambient light, and so you end up with black. In reality you usually end up with a muddy brown colour, therefore a fourth pure black ink is often used. This is called the Key colour, and is why this colour system is abbreviated to CMYK. (It also avoids confusion since B means blue in the RGB system).
Here are two colour charts I am sure you are familiar with from art classes when you were in school.
 |
 |
|
RGB - the additive primary colours. When these three colours of light are mixed together you get white. |
CMY - the subtractive primary colours. When paints or inks of these three are mixed together, they give black. |
|
But teacher told me the primary colours are red, blue and yellow
Prior to modern scientific colour theory, the three subtractive primaries were red, blue and yellow. If you accept the definition that primary colours are a set of colours that can be combined to make a much wider range of colours, then RBY does constitute a set of primary colours. However even Leonardo Da Vinci realised as early as 1500 that these three primary colours were very limited. Using red, blue and yellow as the three primary colours leads to a very limited colour range and made it near impossible to mix a nice bright green, magenta or cyan. This is why many artists considered green as their fourth primary colour.
Some painters throughout the 18th to early 20th century that wanted to stick with three primary colours found that they could mix a much wider range of colours changing their blue for cyan and their red for magenta. It is this system that has evolved into the CMYK system used in colour printers today. |
It is always useful to have a colour wheel handy when working on colour images, especially if you are making changes to the colour balance. For example, you can reduce the amount of one colour in an image by increasing the colour opposite to it on the colour wheel. Or you can increase one colour by increasing the ones either side of it. For example if you have an RGB image and you want to increase the cyan, you can achieve this by either decreasing the red (the colour opposite), or increasing the green and blue (the two primary colours either side).
Colour Models and Colour Spaces
Image files contain lots of numbers which are used to define the colour of each pixel, however these numbers on their own do not represent specific colour. It is only when the numbers are considered in conjunction with a particular colour model that they represent an actual colour.
The two coloured squares show the colour represented by the values (40,198,130) however as you can see the two squares are different colours. This is because the numbers mean a completely different thing when applied in different colour models (the one on the left is the RGB model, the one on the right is the HSB model).
We have already come across the RGB and CMY models in the previous section. The RGB model is by far the most common one the three numbers that describe each pixel define the amount of red, green and blue at that pixel. This model is used by all computers to control the display. It is also the standard model for JPEG files that come straight off your digital camera. Most editing that you will want to do can be done easily in this mode.
However RGB isn't the only colour model - the table below lists some popular ones.
| Model |
Description |
| RGB |
Red Green and Blue - see above |
| CMYK |
Cyan Magenta Yellow Black. Used in the printing industry as these are generally considered the four best subtractive primary colours. Each of the Cyan, Magenta and Yellow inks works by removing one of the main primary colours - i.e. cyan absorbs red light but not green and blue ; magenta absorbs green and yellow absorbs blue. It is far from perfect though which is why some modern photo printers use additional colours. |
| HSB |
Hue, Saturation and Brightness
Hue is a measure of where a colour lies on the standard colour wheel (which is shown along the bottom of the dialog box in a linear manner). In everyday use, hue is identified by the colours name (e.g. red, blue, mauvey shade of pinky russet).
Saturation is the purity of the colour - or the amount of grey in the image. 0% indicates no colour (just grey) and 100% is fully saturated.
Lightness is used to add white or black to the colour |
| CIE L*a*b* |
See section below on colour modes. |
A colour space is a variation on a colour model and has a specific colour gamut (the range of colours that can be represented). For the RGB colour model, the sRGB and Adobe-RGB colour spaces are the two most common. The diagram below shows what is called a CIE Chromaticity diagram. It shows the position within the visible spectrum of the three primary colours in the AdobeRGB and the sRGB colour spaces. You can see that AdobeRGB has a larger colour gamut - however unless all devices and software support the AdobeRGB colour space, you will run into problems. It is generally best to stick to sRGB as this is supported by nearly all monitors and printer drivers.
Every output device has its own colour space and is only able to reproduce colours in its gamut. For example a particular combination of red green and blue may look different when displayed on an LCD screen and a printer because their colour spaces are different. To try and minimise these differences, software like Photoshop carefully manages colour using standard colour spaces like sRGB. All devices that conform to sRGB should interpret a given RGB value in the same way.
Colour Modes (Photoshop)
Photoshop offers different colour modes that you can choose to work in - some of which are directly related to a specific colour model described in the previous section.
| Mode |
Description |
| RGB |
Based on the RGB colour model - it assigns a number (between 0 and 255 for 8 bit per channel images) which represents the amount of each of the three additive primary colours. The numbers are often written in brackets and always in the order red, then green, then blue.
For example (255,255,0) is 100% red, 100% green and no blue which would give bright yellow.
As there are three channels, and each one is 8-bits, it takes 24 binary bits (3 bytes) of information per pixel. Photoshop also supports 16 and 32 bits per channel (48 and 96 bit images respectively).
The precise range of colours possible is dependent on the RGB colour space used.
|
| CMYK |
As mentioned previously, these are the subtractive primaries. Each colour is represented as a percentage (lighter colours having a lower percentage value). Pure white is when all channels have a value of 0%.
As a colour model it is rarely used except when working with scanned images from professional scanners.
|
| LAB Colour |
Based on the CIE L* a* b* colour model a system itself based on how we humans perceive colour. Lab describes how a colour looks, and not how it is mixed from a set of primary colours. Many systems that require precise colour management (such as Photoshop) will use Lab colour as a reference when converting between other colour models as it is considered device independent.
The L stands for lightness and has a value in the range 0 to 100
The a channel represents the red-green axis and has a range of -128 to +127
The b channel represents the blue-yellow axis and has the same range as the a channel
LAB colour is device independent. This means that an image in Lab mode describes what a color looks like under rigidly specified conditions; it's up to Photoshop, or your color management software, to decide what RGB or CMYK values are needed to create that color on your chosen output device.
|
| Greyscale |
A single channel colour mode which represents just the luminance of each pixel and no colour information. In converting from a full colour model to greyscale the computer will calculate the luminance of each pixel. Other than RGB, this is the only other colour model supported by the JPEG file format. As it contains only a single channel, the files are around a third the size of an RGB image.
|
| Indexed |
Specialist mode not covered on this website. |
| Duotone |
Specialist mode not covered on this website. |
The mode determines the number of colours available, the number of colour channels, and also which tools and file formats are available to you. For example if you are working in Lab Color, you won't be able to save your image as a JPEG file without switching back to RGB or Greyscale mode. You can however load a JPEG image into Photoshop and then switch to Lab Colour mode.
|
Photographs
This is a site about photography so I'm sure you are expecting to see plenty of pictures.
For now, why not take a peek at the flickr galleries belonging to the two authors of this site.
Colin's Flickr Page
Phil's Flickr Page
"Classic Quote"
- Quote by
|
