Colour spaces

What they are:

A device color space simply describes the range of colors, or gamut, that a camera can see, a printer can print, or a monitor can display. Editing color spaces, on the other hand, such as Adobe RGB or sRGB, are device-independent. They also determine a color range you can work in. Their design allows you to edit images in a controlled, consistent manner. A device color space is tied to the idiosyncrasies of the device it describes. An editing space, on the other hand, is gray balanced — colors with equal amounts of Red, Green, and Blue appear neutral. Editing spaces also are perceptually uniform; i.e. changes to lightness, hue, or saturation are applied equally to all the colors in the image.

The Additive RGB Colors
The cone-shaped cells inside our eyes are sensitive to red, green, and blue. We perceive all other colors as combinations of these three colors. Computer monitors emit a mix of red, green, and blue light to generate various colors. For instance, combining the red and green "additive primaries" will generate yellow. The animation below shows that if adjacent red and green lines (or dots) on a monitor are small enough, their combination will be perceived as yellow. Combining all additive primaries will generate white.

The Additive RGB Color Space


The Subtractive CMYk Colors
A print emits light indirectly by reflecting light that falls upon it. For instance, a page printed in yellow absorbs (subtracts) the blue component of white light 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, hence the term "CMYk" color space, with k standing for the last character of black.

The Subtractive CMYk Color Space

The LAB and Adobe RGB (1998) Color Spaces
Due to technical limitations, monitors and printers are unable to reproduce all the colors we can see with our eyes, also called the "LAB" color space, symbolized by the horseshoe shape in the diagram below. 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. There are many types of CMYk, depending on the device. From the diagram you can see that 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. This will allow for prints with a wider range of colors. However, most monitors are only able to display colors within sRGB.