Visible light corresponds to a range of wavelengths of electromagnetic radiation from about 400 for red to 800 nanometers for indigo.
In chemistry, the composition of a substance may be deduced by the way in which light is absorbed by it. When light passes through a substance and is passed through a prism and thus divided into its component wavelengths, we may notice darker bands which denote absorption of a specific frequency. Different substances have different characteristic absorption bands. This method of analysis is called spectroscopy.
In astronomy, the spectra of colors of light emitted by stars displays characteristic absorption bands corresponding to the presence of hydrogen, helium and other elements on the way of the beam. These bands may appear shifted towards higher or lower wavelengths, depending on the relative motion of the source and observer. These bands do however retain their relative positions in the spectrum of the light analyzed. The observation that light from more distant sources tends to be shifted further towards red, that is the source appears to be moving away from the observer at a higher velocity, has been the initial impulse for the theory of an expanding cosmos. See Big Bang.
Colors are associated to many different concepts of esoteric study. From among these, we can mention:
Chakras - In Eastern esotericism, the chakras are each traditionally associated to a color. The colors range from red for the first to indigo for the 6th, the 7th is usually described as alternating white and violet.
The channeled source Ra discusses colors in relation to chakras and densities. Each chakra and density would have a 'true' color, corresponding to a precise wavelength of the light spectrum. The composition of a soul could be seen in terms of these colors, a bit as in spectroscopy but now applied at a metaphysical level. We understand that these statements do not refer to the human plane of perception and are thus more symbolic than literal.
Goethe has introduced the circle of colors, where the usually linearly represented spectrum is bent around the circumference of a circle. There, between the adjoining reds and violets is the color magenta, which looks like a smooth bridge from violet to red. Goethe's theory of colors is more geared towards perception of colors by the human than to their physical properties.
Colors are often associated to octaves. Isaac Newton noticed and published a correspondence between the the ratios of the frequencies of themusical notes and the ratios of wavelengths of visible light. The correspondences are as follows:
Do - C 1 : 1 4.40 x 10'4 Red
Re - D 9 : 8 4.95 x 10'4 Orange
Mi - E 5 : 4 5.5 x 10'4 Yellow
Fa - F 4 : 3 5.86 x 10'4 Green
Sol- G 3 : 2 6.60 x 10'4 Blue
La - A 5 : 3 7.33 x 10'4 Indigo
Ti - B 15 : 8 8.25 x 10'4 Violet
Do 2 : 1 Invisible
Gurdjieff discusses octaves extensively. He draws a parallel between colors and musical notes in his descriptions of experiments concerning the Law of Seven in Beelzebub's Tales to his Grandson. Gurdjieffian cosmology sees the universe as composed of vibrations in different media, proceeding at different rates but always according to the Law of Seven or the octave.