Introduction to Colour theory

White colour is composed of three primary colours: red, green and blue. These primary colour are known as additive colour, make out the entire colour that we see in the TV and computer monitor. Subtract any one of these colour and you have the remaining colour which consists of cyan, magenta and yellow. The combination of these three colour will make up black. CMK is normally found in dyes or pigment in our inks on our white paper. Below is an example of the RGB and CMY colour. Take note at the overlapped section of RGB which are cyan, magenta and yellow. The same is goes for CMK.

Image from Mark Boulton
Colour wheel
By putting all these six colours together, you will have a colour wheel. Red, green and blue are found at 120^o apart. The same goes for cyan, yellow and magenta. In colour correction, you use the opposite colour to cancel each other out. For example, if you have more blue on the picture, you may want to eliminate by adding more yellow to cancel the effect of blue.


What do we need to know colour?
The human eye and brain will adjust our perception of white whatever the colour quality of the illuminating light source is. To the eyes, a white paper will appear white whether we are reading them in the daylight or in tungsten light source. Digital cameras do not have such ability to tell accurately, therefore you will need to manually set the camera or manipulate the colour in the photoshop. There are preset white balances to handle some common lighting scenario. However, they can only handle from warm (red) to cool (blue). Fortunately, any unwanted colour cast such as green, magenta can be corrected either by using the colour correction filters or to use the saturation/hue in the Photoshop. This will help you to render your colour to be as true colour as possible. Knowing the colour wheels helps us to manipulate light and to filter any incorrect colour cast result from a mixture of light source. You can either choose to remove any unwanted colour cast in the images or you can add in colour like magenta to create a funky image.