Therefore, if viewing conditions vary, the XYZ colour model is not sufficient, and a colour appearance model is required to model human colour perception.
==ColorColour appearance parameters==
The basic challenge for any colorcolour appearance model is that human colorcolour perception does not work in terms of XYZ tristimulus values, but in terms of '''appearance parameters''' ([[hue]], [[lightness]], [[brightness]], [[colorfulnesscolourfulness|chroma, colorfulnesscolourfulness and saturation]]). So any colorcolour appearance model needs to provide transformations (which factor in viewing conditions) from the XYZ tristimulus values to these appearance parameters (at least hue, lightness and chroma).
