To be able to describe colours in terms of their characteristics, we need a colour model, e.g. the CMYK colour model, RGB colour model, LCH/HSB colour model, CIELUV or the CIELab colour model. Colours of this colour model can be described by numerical values. In the CMYK model, a certain colour can be described in screen values, while the values L, a and b define colours in the Lab colour space. CMYK and RGB colour models are so-called device dependent colours, as a CMYK (ISOCoated, Swap, Toyo) or RGB colour space (Adobe-RGB, ECI-RGB, sRGB) does not exist. In contrast, Lab describes colours absolutely.