It is important to not confuse color modes with color spaces. The former defines the general color model being leveraged as part of your current workflow, while the latter describes a specific set of color instructions that fall within that color model. The following sections describe some of the most common color modes and several common color spaces and their typical uses.
RGB stands for Red, Green, Blue. This color mode is used by all monitors, and works by combining specific amounts of red, green, and blue per pixel to create an image. It is designed for mediums that project rather than reflect light. Also called additive color, because the more red, green, and blue you add together, the closer the displayed value gets to pure white.
It is generally a good idea to edit raster images in RGB mode whenever possible, unless the image was provided to you in CMYK mode and is destined for press output, in which case it is better to leave the image in CMYK mode as you edit.
CMYK stands for Cyan, Magenta, Yellow, Black. This is the color model used by four-color printing presses and is also the basis for inkjet printing systems. It is designed for physical mediums like paper. Its colors are created when certain components of light are absorbed (or subtracted) by the physical medium; the remaining colors that we see are those that were reflected. For this reason CMYK is often called "subtractive color."
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