Here we will look in greater detail at primary, secondary and tertiary colours.
Figure 1 There are three primary colours of Red, Green and Blue (RGB). As these are mixed they form lighter colours, and when all three are mixed together they appear as white.
Humans see RGB as the primary colours because we evolved that way; our eyes are sensitive to, and respond to, this set of colours. But not all areas of the graphics industry choose to work in these colours.
Figure 2 Artists and graphic designers, who work with paintings and illustrations more than they do with photographs, often refer to Red, Yellow and Blue as the primary colours. They will use a colour wheel which shows the relationship between the varying hues.
As yellow is such a bright colour it is seen as more important than green, but it does cause some problems in mixing several of the other colour values. However, even if this is useful to artists it has no value for correcting digital images and so is not covered in the following notes.
Figure 3 Printers often refer to the printing colours of Cyan, Magenta and Yellow as primary colours, with Black being added to give a total of the four printing inks of CMYK.
Technically the CMY inks are complementary to RGB; they are secondary colours as they are a mix of the primary RGB colours. But as printers always work with CMYK it is therefore natural to think of these inks as being the primary colours instead.
Secondary and tertiary colours
Figure 4 Like the primary RGB colours, the secondary colours consist of just three hues, namely Cyan, Magenta and Yellow.
Primary colours are just single colours with no other colour mixed in. For example, a Red can become darker or lighter to form variations of Red, but it still just consists of the one hue. Secondary colours are a mix of any two primary colours, such as Yellow being a mix of Green and Red. But these secondary colours can also vary in strength from dark to light. For example, Yellow becomes brown as it gets darker, but it remains a mix of the two primary colours Green and Red, and so it is still a secondary colour.
However, this is to think of colours in a pure form, whereas in real life these colours are never that clean. A fire engine may be Red, but it is never a pure red. Under natural light all the three primary colours are reflected by the fire engine; the engine is still red but it is only predominately so as there are still Green and Blue values present.
Figure 5 Tertiary colours are formed by mixing the thre primary colours, or any two of the secondary colours, which amounts to the same thing.
Since all colours under natural light must have RGB values, it is therefore useful to think of colours in digital imaging as being tertiary colours. If the example of the fire engine was found to be pure red, then it would be an impossible colour, a colour not found in nature. There are exceptions though. A red light is so bright compared to its surrounding colours that the Green and Blue values are too low to register.