Understanding Color Modes

In Photoshop, color modes determine (a) the number of colors and channels, as well as file size, (b) what colors the eye sees and how they are mixed, (c) the working color space of the active document, and (d) color space of the output document.

Different output devices (printers, computer monitors, etc.) use different color mixes. Therefore, the method of document output (print, press, or monitor) determines the color mode you will use for a document. Each color mode has a preset number of channels that, in Photoshop, cannot be changed.