Device-dependent models
The RGB value (200, 80, 40) sent to two different monitors gives two different colors. The Lab value (49.7, 45.8, 46.3) describes one specific color, whatever you display it on. That difference splits color models into two groups.
Model versus color space
A color model is a notation: what the components are and how they combine. RGB only says a color is three light intensities. It does not say which lights.
A color space is a model tied precisely to the colors people see, that is, to CIE XYZ. An RGB space needs three things:
- the primaries (xy coordinates of red, green and blue),
- the white point,
- the transfer function, i.e. the gamma curve.
sRGB, Adobe RGB and Display P3 are three different spaces built on the same RGB model.
Two groups of models
| Device-dependent | Device-independent |
|---|---|
| RGB: light intensities of a particular screen | CIE XYZ: the response of the standard observer |
| CMYK: ink amounts of a particular print process | CIELAB, CIELUV: XYZ reshaped to match perception |
| HSV, HSL, HSI: transformations of RGB | xyY: chromaticity and luminance |
| YUV, YIQ, YCbCr: RGB transformations for image transmission | Oklab, CAM16: newer perceptual models |
Models in the left column only become exact once you know which space they belong to. HSV from sRGB and HSV from Display P3 are two different colors with the same numbers.
Color gamut
Every device can reproduce only part of the visible colors. That range is its gamut. On the chromaticity diagram an RGB display gamut is a triangle, while a CMYK printer gamut is an irregular polygon that reaches beyond sRGB in some places (some cyans) and is much smaller in others (vivid blues, greens, oranges).
When a source color lies outside the destination gamut, it has to be replaced by the nearest possible one. The CIE XYZ and RGB page has an example of a color that needs negative red in sRGB.
Connecting them
For a color to look similar on different devices, the conversion has to pass through an independent space:
Each device is described by an ICC profile that states how its values relate to XYZ or Lab. Details are in the article on color management.
The calculator on this page assumes RGB values are in sRGB. That is a sensible default for web graphics, but not for camera photos in Adobe RGB or files from P3 displays.