A spectral color is a color that is evoked by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths. Every wavelength of light is perceived as a spectral color, in a continuous spectrum; the colors of sufficiently close wavelengths are indistinguishable.
Indigo is often omitted as simply a shade of blue/violet, and cyan was not included historically. The first division was by Newton, in his color wheel, and he used Red, Orange, Yellow, Green, Blue, Indigo and Violet; a mnemonic is ROYGBIV.
Among some of the colors that are not spectral colors are:
Visible light is a blend of (normally) all spectral colors, leading to a common understanding of visible light as "white"; however, depending upon the specific light source, the relative intensity of each color varies. This phenomenon explains why there exists a tinge of color perceived in certain light sources themselves. For example, high pressure sodium and some fluorescent sources have an off-white color. This phenomenon is important in evaluating the effects of light upon humans from light sources that are not matched well to the spectrum of natural sunlight. The study of these health effects is dealt with in the field of over-illumination.