1.4.2 The CMY Color Model

This stands for cyan-magenta-yellow and is used for hardcopy devices. In contrast to color on the monitor, the color in printing acts subtractive and not additive. A printed color that looks red absorbs the other two components $G$ and $B$ and reflects $R$. Thus its (internal) color is G+B=CYAN. Similarly R+B=MAGENTA and R+G=YELLOW. Thus the C-M-Y coordinates are just the complements of the R-G-B coordinates:

$$\begin{pmatrix} C \\ M \\ Y \end{pmatrix}= \begin{pmatrix} 1 \\ 1 \\ 1 \end{pmatrix}- \begin{pmatrix} R \\ G \\ B \end{pmatrix}$$

If we want to print a red looking color (i.e. with R-G-B coordinates (1,0,0)) we have to use C-M-Y values of (0,1,1). Note that $M$ absorbs $G$, similarly $Y$ absorbs $B$ and hence $M+Y$ absorbs all but $R$.

Black ( $(R,G,B)=(0,0,0)$) corresponds to $(C,M,Y)=(1,1,1)$ which should in principle absorb $R$, $G$ and $B$. But in practice this will appear as some dark gray. So in order to be able to produce better contrast printers often use black as $4^{\text{th}}$ color. This is the CMYK-model. Its coordinates are obtained from that of the CMY-model by $K:=\max(C,M,Y)$, $C:=C-K$, $M:=M-K$ and $Y:=Y-K$.