What is Colour? - Physics and Perception

What you need to know

Colour does not exist as a physical property of an object. It arises as a perceptual phenomenon through the interplay between four elements: light, object, observer and context.

Different light sources emit light with varying spectral compositions, which directly affects colour perception. In professional colour assessment, three light source properties play a leading role: the spectral power distribution (SPD), colour temperature and the colour rendering index (CRI).

Nearly all colour systems use three primary colour characteristics: hue (what we mean by “colour”), saturation (intensity/purity) and lightness (how light or dark).

Reliable colour assessment requires standardised conditions. These include specific requirements for lighting, assessment environment, sample preparation and observation method.

Individual differences in colour perception play an important role in professional situations. Approximately 8% of men and 0.5% of women have some form of colour vision deficiency.

Business impact

Research shows that colour determines 60-80% of consumers' initial reaction to garments.

Reformulating a car paint colour can cost up to €100,000, while production stoppages due to colour problems at major car manufacturers can cost up to €30,000 per hour.

Approximately 8% of men and 0.5% of women have some form of colour vision deficiency, with direct consequences for professional colour assessment.

Unattractive colour in vegetables and fruit is a major cause of food waste by consumers, costing billions of euros annually.

1.1 Definition and physical basis of colour

Colour is everywhere. It is one of our most direct ways of experiencing and interpreting the world around us. But what exactly is colour? When we delve deeper into this seemingly simple question, we arrive at a surprisingly complex reality.

Colour is not a physical property of an object, such as mass or size. Colour is a perceptual phenomenon that arises as the result of the interaction between four fundamental components: light, object, observer and context (see figure 1). ^{[^1]}

This means that without light no colour exists, without an object no colour is reflected, without an observer no colour experience takes place, and without context no meaningful interpretation is possible.

“Colour is not a physical property of an object, such as mass or size. Colour is a perceptual phenomenon that arises as the result of the interaction between four fundamental components: light, object, observer and context.”

In a professional context, this means that for accurate colour assessment all four components must be controlled or at least well documented. ^{[^2]}

1.1.1 The physical basis of colour

From a physics perspective, light consists of electromagnetic radiation. The visible spectrum for the human eye covers only a small part of the electromagnetic spectrum, roughly between 380 and 780 nm wavelength (see figure 2).

Within this limited range, we perceive different wavelengths as different colours:

380-450 nm: Violet

450-485 nm: Blue

485-500 nm: Cyan/Blue-green

500-565 nm: Green

565-590 nm: Yellow

590-625 nm: Orange

625-780 nm: Red

The boundaries between these colours are not sharp, but gradual. It is important to understand that colour is not an inherent property of light itself — light merely has a wavelength or a spectrum of wavelengths. Colour only arises when our visual system interprets this light. ^{[^3]}

“It is important to understand that colour is not an inherent property of light itself — light merely has a wavelength or a spectrum of wavelengths. Colour only arises when our visual system interprets this light.”

1.1.2 Light as the basis for colour perception

Light forms the foundation of all colour perception. In doing so, it fulfils three essential functions:

1. Carrier of colour information:

Light transports colour information from objects to our eyes. The spectral composition of this light determines which colour information is available for perception.

• Reference frame:

Our visual system uses the general lighting conditions as a reference frame for colour assessment. This allows us to perceive colours as relatively constant under different lighting conditions, a phenomenon known as colour constancy.

• Stimulus for the visual system:

Light activates the photoreceptors in our retina, which is the beginning of the visual perception process.

Light sources and their characteristics

Different light sources produce light with different spectral compositions, and these differences have a direct influence on colour perception. For professional colour assessment, several light source properties are important: ^{[^10]}

Spectral Power Distribution (SPD)
The spectral power distribution of a light source describes the relative intensity of radiation at different wavelengths. This is the most fundamental property for colour assessment because it directly determines which wavelengths are available for colour perception.