CMY / CMYK

The subtractive ink color model -- cyan, magenta, yellow, plus a separate black channel extracted from what all three share.

CMY is subtractive (ink absorbs, rather than adds, light): $C=1-R$ , $M=1-G$ , $Y=1-B$ on normalized channels. CMYK adds a fourth, black channel $K=\min(C,M,Y)$ , with the remaining $C'=(C-K)/(1-K)$ (and likewise $M'$ , $Y'$ ) carrying only the leftover color the black ink cannot. RGB's gamut and CMYK's gamut are different shapes -- some RGB-representable colors have no exact ink equivalent, independent of any calibration error.