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An ICC profile defines the bidirectional conversion between a neutral "profile connection" color space (CIE XYZ or Lab) and a selected colorspace, in this case both RGB and CMYK. The precision of the conversion depends on the profile itself, the exact methodology, and because the gamuts do not generally match, the rendering intent and ...
To see how this works in practice, suppose we have a particular RGB and CMYK color space, and want to convert from this RGB to that CMYK. The first step is to obtain the two ICC profiles concerned. To perform the conversion, each RGB triplet is first converted to the Profile connection space (PCS) using the RGB profile.
Common color spaces based on the RGB model include sRGB, Adobe RGB, ProPhoto RGB, scRGB, and CIE RGB. CMYK uses subtractive color mixing used in the printing process, because it describes what kind of inks need to be applied so the light reflected from the substrate and through the inks produces a given color. One starts with a white substrate ...
Variations in paper and ink processes account for the various combinations of non-invertible matrices and look-up tables used in converting from RGB to CMYK. If you wish to obtain CMYK values for these colors, you will get appropriate conversion values from your graphics application when you define the type of ink and paper you will be printing on.
In order to convert RGB or CMYK values to or from L*a*b*, the RGB or CMYK data must be linearized relative to light. The reference illuminant of the RGB or CMYK data must be known, as well as the RGB primary coordinates or the CMYK printer's reference data in the form of a color lookup table (CLUT).
The official RGB values in the document must not be used, because they are noticeably oversaturated, in addition to never being used in practice by either IdFM or RATP. Instead, use RGB values that were generated by converting official CMYK values in Adobe Illustrator, under the European General Purpose 3.1 colour profile. [a]
Care must be taken when using electronic design programs (e.g. when managing a CMYK document in Adobe Illustrator or Corel Draw) – "black" may or may not equal 100%K depending on the CMYK profile specified in the image's settings, and Photoshop will represent the various tones using RGB values close to black; in an RGB document, "black ...
In the CIE 1931 model, Y is the luminance, Z is quasi-equal to blue (of CIE RGB), and X is a mix of the three CIE RGB curves chosen to be nonnegative (see § Definition of the CIE XYZ color space). Setting Y as luminance has the useful result that for any given Y value, the XZ plane will contain all possible chromaticities at that luminance.