Color glossary
Plain-language definitions of color terms used in Close Hue and across design.
Updated
ΔE (Delta E)
ΔE (delta-E) is a numerical measure of how different two colors look to the human eye. ΔE ≈ 0 means identical; ΔE ≈ 2 is the threshold of just-noticeable difference for a trained observer; ΔE ≈ 10 is clearly different but still in the same hue family. The formula was standardized by the CIE (International Commission on Illumination); the modern version is CIEDE2000. Close Hue uses ΔE to score guesses.
CIEDE2000
CIEDE2000 (full name: CIE 2000 color-difference formula) is the latest CIE-standardized formula for perceptual color difference, published in 2001. It improves on its predecessors (CIE76, CIE94) by adding hue-, chroma-, and lightness-dependent weighting that better matches human perception. The reference implementation notes are in Sharma, Wu, & Dalal (2005). Used by Close Hue's scoring; available as a standalone tool at /tools/delta-e-calculator.
CIE Lab (L*a*b*)
CIE Lab is a color space designed by the CIE to be approximately perceptually uniform — equal numerical distances correspond roughly to equal perceived differences. L* is lightness (0–100); a* and b* are color-opponent axes (red-green and blue-yellow, each unbounded). CIEDE2000 computes color difference inside Lab space. Lab itself is derived from XYZ, which is derived from linearized RGB.
sRGB
sRGB is the standard RGB color space defined by HP and Microsoft in 1996, adopted as the default for the web. When you write a hex code like #3B82F6, you're naming a color in sRGB. sRGB uses a gamma curve for storage; perceptual color math (CIE Lab, OKLab, luminance) requires linear RGB, which is sRGB with the gamma curve removed.
Linear RGB
sRGB stores colors on a gamma curve (roughly value^2.2) so that 8-bit color values pack more precision into the dark midtones, where the human eye is more sensitive. To do correct color math — averaging, luminance, perceptual conversions — you first gamma-decode to "linear" RGB, where channel values are proportional to physical light intensity. Forgetting this step is the most common bug in DIY color tools.
OKLab
OKLab is a perceptually-uniform color space designed by Björn Ottosson in 2020, intended as a more accurate and easier-to-use replacement for CIE Lab. Like Lab, equal distances correspond roughly to equal perceived differences — but the math is simpler and the predictions match human perception better, especially in blues. Used by Close Hue's color blending and tonal-scale tools.
OKLCH
OKLCH is the cylindrical-coordinate version of OKLab — same color space, different parametrization. L is lightness (0–100%); C is chroma (saturation, 0+, typically up to ~0.4 for in-gamut sRGB); H is hue in degrees (0–360°). CSS Color Module 4 added the oklch() function in 2022, and Tailwind v4 uses OKLCH internally for its color system. Convert any color to OKLCH at /tools/hex-to-rgb.
HSL (Hue, Saturation, Lightness)
HSL is a cylindrical reorganization of RGB into Hue (0°–360° around the color wheel), Saturation (0–100%, gray to vivid), and Lightness (0–100%, black to white). It's much easier than RGB for picking related colors — e.g. rotating Hue by 180° gives the complementary color — but it is not perceptually uniform: a yellow at 50% lightness looks much brighter than a blue at 50% lightness because HSL lives inside the same gamma curve as sRGB.
HSV / HSB
HSV (Hue, Saturation, Value) — also called HSB (Hue, Saturation, Brightness) — is another cylindrical RGB model. The hue axis is the same as HSL; saturation and brightness are computed differently. Most color pickers expose HSB, including Close Hue's in-game picker. Like HSL, it's not perceptually uniform.
Gamut
A color gamut is the subset of all visible colors that a particular color space, display, or printing process can reproduce. sRGB's gamut covers about 35% of the visible spectrum; modern wide-gamut spaces like Display P3 cover ~50%; the human eye can see far more than any current display. Colors "out of gamut" for sRGB cannot be displayed by typical web browsers and get clipped to the nearest in-gamut value.
Perceptual uniformity
A color space is perceptually uniform if equal numerical distances between two colors correspond to equal perceived differences. RGB and HSL are not perceptually uniform — moving 10 units in green looks like a much bigger change than moving 10 units in blue. CIE Lab and OKLab are designed to be approximately uniform, which is why ΔE and color-blending math operate in those spaces rather than RGB directly.
Dichromacy
Dichromacy is a form of color vision deficiency in which one of the three cone types (long, medium, or short wavelength) is absent. The three dichromatic conditions are protanopia (no L cones), deuteranopia (no M cones), and tritanopia (no S cones). Together they affect about 2% of men and far fewer women. Try the color blindness simulator at /tools/color-blindness-simulator.
Trichromacy
Trichromacy is the standard human color vision system, with three types of cone photoreceptors sensitive to long (~564 nm, red), medium (~534 nm, green), and short (~420 nm, blue) wavelengths. Together they let the brain reconstruct millions of distinguishable colors from three signals. About 92% of humans have typical trichromacy; the rest have anomalous trichromacy or dichromacy.
Anomalous trichromacy
Anomalous trichromacy is a partial form of color vision deficiency: all three cone types are present and functional, but one has its peak sensitivity shifted, weakening discrimination in that part of the spectrum. The three types are protanomaly, deuteranomaly, and tritanomaly. More common than full dichromacy, often milder. Their visual experience interpolates between typical trichromacy and the matching dichromatic condition.
Achromatopsia
Achromatopsia is a rare condition (~0.003% of population) in which all three cone types are absent or non-functional. The person sees only in shades of grey, perceiving brightness via rod cells alone. Often accompanied by light sensitivity and reduced visual acuity.
WCAG contrast ratio
The WCAG (Web Content Accessibility Guidelines) contrast ratio measures how different two colors are in relative luminance, on a scale from 1:1 (identical) to 21:1 (pure black on pure white). WCAG 2.1 requires 4.5:1 for normal body text (Level AA), 3:1 for large text or UI components, and 7:1 for enhanced contrast (AAA). Check any pair at /tools/contrast-checker.
Relative luminance
Relative luminance is the WCAG-defined measure of a color's perceived brightness, ranging from 0 (black) to 1 (white). It's computed by linearizing each sRGB channel and combining them with the weights 0.2126·R + 0.7152·G + 0.0722·B. Green dominates because the human eye is most sensitive to green wavelengths; blue contributes the least. Two colors' contrast ratio is computed from their relative luminances.
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The glossary covers terms we use elsewhere on the site. If we mention a concept somewhere and it isn't defined here, that's a gap worth filing — let us know.