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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. ). 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.
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, since Additive colors.]]
In which include all, or most spectral colors, they form a part of boundary of the set of all real colors. When considering a three- 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 , 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 color 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 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 Infrared or ultraviolet.
Spectral colors are universally included in color science 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.
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. Isaac Newton color terms included red, orange, yellow, green, blue, indigo, and violet; this color sequence is still used to describe spectral colors colloquially and a mnemonic for it is commonly known as "Roy G. Biv".
In modern divisions of the spectrum, indigo is often omitted and a blue-green color is sometimes included. Some have argued that Newton's indigo would be equivalent to our modern blue, and his blue equivalent to our blue-green. However, his nonintuitive choices can be better explained. In the table below, note how wavelength is not proportional to hue (which is approximately perceptually uniform). Color systems such as ISCC-NBS attempt to divide the spectrum into sections that appear perceptually uniform. On the other hand, Newton's sections are approximately uniform in size as they would have physically appeared in the diffracted spectrum, i.e. each about 40nm "wide". In this theory, the sections were divided without influence of his own perception, and each section was then given a name that best suited its average color. In contrast, the sections in the ISCC-NBS spectrum vary greatly in wavelength range, but are more consistent in the hue degree range. Both instances deviate from the basic color terms used in English, only some of which are spectral colors.
The table below includes several definitions where the spectral colors have been categorized in . The hue that a given monochromatic light evokes is approximated at the right side of the table.
+Spectral color classifications
!scope="col" wavelength
!scope="col" Isaac Newton
!scope="col" ISCC-NBS
!scope="col" Malacara (2025). 9780819483973, SPIE. ISBN 9780819483973
!scope="col"CRC Handbook (2025). 9780849332500, CRC Press. ISBN 9780849332500
!scope="col"Hue 380 Violet Violet Violet Violet 250° 390 250° 400 250° 410 249° 420 249° 430 Blue 249° 440 Blue 247° 450 Blue Blue 245° 460 242° 470 238° 480 226° 490 Green Blue-Green 190° 500 Green Cyan Green 143° 510 126° 520 Green 122° 530 Yellow 117° 540 113° 550 Yellow-Green 104° 560 93° 570 Yellow Yellow 62° 580 Orange Yellow Orange 28° 590 Orange Orange 14° 600 7° 610 Red 5° 620 Red Red 3° 630 Red 2° 640 1° 650 1° 660 1° 670 0° 680 0° 690 0° 700 0° 710 0° 720 0° 730 0° 740 0° 750
Extra-spectral colors
Among some of the colors that are spectral colors are:
Notes
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