A loaded question with not one answer!
Color control has historically been done by visual inspection of samples and comparison to a standard with the disadvantages of being subjective. For more objectivity, the internationally agreed upon CIELAB system is used since 1976. Meanwhile, several modifications of the CIELAB color equation like ΔE2000 were introduced for solid colors. For effect colors, the development even goes one step further with the introduction of the ΔE DIN6175. The goal of all modifications was to improve the correlation between visual perception and instrumentally measured values.
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