thumb|A [[rainbow is a decomposition of white light into all of the spectral colors.]]
thumb|Laser beams are monochromatic light, thereby exhibiting spectral colors.
A spectral color is a color that is evoked by monochromatic light, i.e. either a spectral line with a single wavelength or frequency of light in the visible spectrum, or a relatively narrow spectral band (e.g. lasers). Every wave of visible light is perceived as a spectral color; when viewed as a continuous spectrum, these colors are seen as the familiar rainbow.
Non-spectral colors (or extra-spectral colors) are evoked by a combination of spectral colors.
In color spaces
[[File:CIE1931xy blank.svg|right|thumb|
The spectrum colors are the colors on the horseshoe-shaped curve on the outside of the diagram. All other colors are not spectral: the bottom line is the line of purples, whilst within the interior of the diagram are unsaturated colors that are various mixtures of a spectral color or a purple color with white, a grayscale color. White is in the central part of the interior of the diagram.]]
In color spaces which include all, or most spectral colors, they form a part of boundary of the set of all real colors. When considering a three-dimensional color space (which includes luminance), the spectral colors form a surface. When excluding luminance and considering a two-dimensional color space (chromaticity diagram), the spectral colors form a curve known as the spectral locus. For example, the spectral locus of the CIE xy chromaticity diagram contains all the spectral colors (to the eye of the standard observer).
A trichromatic color space is defined by three primary colors, which can theoretically be spectral colors. In this case, all other colors are inherently non-spectral. In reality, the spectral bandwidth of most primaries means that most color spaces are entirely non-spectral. Due to different chromaticity properties of different spectral segments, and also due to practical limitations of light sources, the actual distance between RGB pure color wheel colors and spectral colors shows a complicated dependence on the hue. Due to the location of R and G primaries near the 'almost flat' spectral segment, RGB color space is reasonably good with approximating spectral orange, yellow, and bright (yellowish) green, but is especially poor in reproducing the visual appearance of spectral colors in the vicinity of central green, and between green and blue, as well as extreme spectral colors approaching IR or UV.
Spectral colors are universally included in scientific color spaces such as CIE 1931, but industrial and consumer color spaces/models such as sRGB, CMYK, and Pantone, do not typically include any spectral colors. Exceptions include Rec. 2020, which uses three spectral colors as primaries (and therefore only includes precisely those three spectral colors), and color spaces such as the ProPhoto RGB color space which use imaginary colors as primaries.
In color spaces such as CIELUV, a spectral color has maximal saturation. In Helmholtz coordinates, this is described as 100% purity.
In dichromatic color spaces
In dichromatic color vision there is no distinction between spectral and non-spectral colors. Their entire visible gamut can be represented by spectral colors.
Spectral color terms
The spectrum is often divided into color terms or names, but aligning boundaries between color terms to a specific wavelength is very subjective.
The first person to decompose white light and name the spectral colors was Isaac Newton, in the 1660s. Early in the study of radiometry, Newton was not able to measure the wavelength of the light, but his experiments were repeated contemporarily to estimate wavelengths where his color term boundaries probably lay. However, a more recent explanation is that Newton was working by analogy with the musical scale.
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!scope="col"|CRC Handbook
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| rowspan="5" |Violet
| rowspan="6" |Violet
| rowspan="5" |Violet
| rowspan="7" |Violet
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Extra-spectral colors
Among some of the colors that are spectral colors are:
- Grayscale (achromatic) colors, such as white, gray, and black.
- Any color obtained by mixing a gray-scale color and another real color (either spectral or not), such as pink (a mixture of a reddish color and white), or brown (a mixture of orange and black or gray).
- Violet-red colors, which include colors in the line of purples (such as magenta and rose), and other variations of purple and red.
- Impossible colors, which cannot be seen under normal viewing of light, such as over-saturated colors or colors that are seemingly brighter than white.
- Metallic colors which reflect light by effect.