Light is energy in the form of electromagnetic radiation in the wavelength range 380nm - 770nm, which is what our visual system is able to perceive.

Colour is the interpretation that makes our visual system of the structural composition of the light detected by the eye. Every illuminated surface absorbs all or part of the electromagnetic radiation and reflects the rest. The colour which we perceive depends on the amount of light that is emitted, reflected, transmitted and absorbed by each wavelength in the visible spectrum.

The human eye only perceives colour when light is abundant. With little light we can see black and white. White colour is the result of the combination of all the colours, while black is the absence of light.

A white light can be separated in all the colours of the spectrum by a prism. In Nature this separation occurs in a rainbow. The temperature of the colour is a parameter which is used to indicate the tone of white light for the comparison of a black body at that temperature. Meanwhile, the chromatic reproduction index indicates the capacity of a light source to reproduce colours of illuminated objects.

Colour is artificially generated by the combination of the primary wavelengths: red, green and blue, keeping in mind the characteristics of our visual system. Colours are classified in groups: warm (yellows and reds) and cold (greens and blues). The CIE (International Commission of Illumination) has defined as a standard of identification the chromatic colour diagram CIE, where each colour is represented by the coordinates X and Y. For objects which are not light emitters, colour can also be generated by absorption, emission or addition of wavelengths.

Colours produce some physiological and psychological effects over people such as:

Sense of warmth: warm colours (from red to yellowish green). They are dynamic, exciting colours that produce a sensation of closeness.
Sense of cold: cold colours (from green to blue). Colours that calm and rest, producing a distancing feeling.
Purity or saturation of colour: the clear colours animate and give the sensation of lightness, while darker colours depress and give a slowing feeling.
Harmony: is produced when two or more colours are mixed with a nice effect.

Recent studies have demonstrated that the colour of Light produces alteration in the cardiac rhythm and facilitates the conciliation of dreams.