A color space (or color model) is an abstract mathematical model describing the way colours can be represented as groups of numbers, typically as three or four values or color components (e.g. RGB and CMYK are color models).
The Additive RGB Colors
The cells inside our eyes (called cone-shaped cells) are sensitive to red, green, and blue. We perceive all other colors as combinations of these three primary ones. Computer monitors emit a mix of red, green, and blue light to generate various colors. For example, combining the red and green “additive primaries” will generate yellow. Combining all additive primaries (red, green and blue) will generate white.
The Subtractive CMYk Colors
A printed page emits light indirectly by reflecting light that falls upon it. For example, a page printed in yellow absorbs the blue component of ight and reflects the remaining red and green components, thereby creating a similar effect as a monitor emitting red and green light. Printers mix Cyan, Magenta, and Yellow ink to create all other colors. Combining these subtractive primaries will generate black, but in practice black ink is used. So the “k” in “CMYk” color space stands for black.
The LAB and Adobe RGB (1998) Color Spaces
Because of the technical limitations, monitors and printers can’t reproduce all the colors we are able to see with our eyes (called the “LAB” color space). The group of colors an average computer monitor can replicate is called the (additive) sRGB color space. The group of colors a printer can generate is called the (subtractive) CMYk color space. Certain colors are not visible on an average computer monitor but printable by a printer and vice versa. Higher-end digital cameras allow you to shoot in Adobe RGB (1998), which is larger than sRGB and CMYk.