Achromatic and chromatic colours

Broadly speaking, colours are divided into achromatic and chromatic colours. The achromatic colours are white and black, and the various shades of grey between them. All the colours that are not achromatic are classified as chromatic colours.

The three qualities of colours

Now let's take a closer look at how colour is expressed. To the left are two red circles. They may appear to be the same shade of red at a glance, but a closer look reveals some differences. First of all, what hue are the two circles? They are both red. What about their brightness? The one on the left is brighter than the one on the right. Finally, what about their intensity? The circle on the left has a brilliant intensity, while the one on the right appears rather dull. This goes to show that two colours that may appear the same at first can appear different on closer examination. To sum it up, colour can be expressed as combinations of hue, brightness, and intensity.

Hue

Apples are red, lemons are yellow, and the sky is blue. When we think of different colours we are often thinking in terms of hue. Hue is the term used for words such as red, yellow, or blue that describe different shades. Red and yellow are completely different hues, but if we mix red and yellow paint together we get orange. Mixing yellow and green gives us yellow green or pea green, and mixing green and blue gives us blue green. Thus, the hues are all related to each other and can be arranged to form a circle. The result is called the "colour circle."

< Conceptual view of colour space based on the L*a*b* model> The L*a*b* model is presently the most popular system used to express the colour of objects. It was standardised in 1976 by the Commission Internationale d'Eclairage (CIE) Under the L*a*b* model, brightness is expressed as "luminance"or L*, while hue and chroma are expressed as a* and b*. The larger the numeric values, the more vibrant and intense the colour, and the closer to zero the numeric values, the duller the colour. The illustration at left is a three-dimensional image of colour space based on the L*a*b* model.

Brightness (Value)

Some colours appear brighter and others appear darker. This is why brightness is one of the qualities used to classify colours. For example, a lemon may appear to be a brighter yellow than a grapefruit. How does the yellow colour of a lemon differ from the red of a good red wine? The yellow colour of the lemon is brighter. In this way, brightness can be used to compare colours with different hues.

Chroma

Consider a lemon and a piece of khaki fabric with the same hue. The difference in the perceived colours of the lemon and the khaki fabric can be accounted for less by the brightness of the lemon's yellow than by its intensity, which differs greatly from that of the fabric. Thus, in addition to hue and brightness, we must add a third quality, intensity, which is also referred to as "chroma."