CMYK refers to cyan, magenta, yellow, and black respectively. This is the traditional four-color semi-transparent ink combination used to reproduce a wide range of colors on printing presses. Cyan is a very light blue. To distinguish between blue and black, the last letter of black (K) is used as the fourth-color designation.
This additive color space is established upon a white base. CMYK colors are mixed to build hues. The absence of all color is white (or the color of the paper). Designers desiring vibrant colors limit ink formulas to CMY. Percentages of colors may be represented as C15, M30, Y90 (sometimes with hyphens or spaces preceding numbers). Theoretically, the combination of 100 percent each of CMY should produce black. Due to ink impurities, it appears brownish gray. So black ink is used to enhance contrast. To prevent smearing and other issues when combining colors, limit total ink coverage to 300% or less.
Attempting to replicate light hues with four colors can result in somewhat darker dots spread farther apart. This distracts from the continuous tone effect. In recent years, inkjet printers have both decreased the dot sizes and added more ink colors to achieve photographic quality.
With older technologies, it has been difficult to achieve photo realistic results from black and white images. This is because light areas contained somewhat distracting tiny dots. The alternative has been to mix a bit of the other colors with black. However, this can result in color shifts or unwanted tints. Newer inkjet printers may use light gray, dark gray, photo black, and matte black to achieve outstanding results from black and white photos.