Color modes are the basis for the representation of a pixel's color value. Between different installations of Photoshop, a document will be interpreted the same way by the basic image processing system. Along with color profiles, color modes determine how an image will be represented on screen or in print.
Color modes have been present in all versions of Photoshop. Lab is the only mode not present since the beginning, and was added in 3.0. At some point, HSB and HSL modes were removed, although remnants of both remain as described below.
Version 2.5 only offered four modes: RGB, Indexed, Grayscale, and "Bitmap".
Color modes were given their own menu, but were incorporated into the Image menu starting with version 4.0.
List of color modes
Every image has a color mode. These modes differ in how many channels are used to represent a color, and how deep each channel is.
Basic color modes
- The most basic mode, consisting of a single channel that maps values to grays, from black to white.
- An 8-bit mode that translates each of the 255 possible color values into an RGB triad. Indexed color mode is not considered to have a channel, but is still useful in certain applications.
A minimum of three channels is needed to represent the majority of colors visible to humans. The act of dividing each color into the different channels is called separation, and the act of combining them is called compositing—these are handled differently by each mode.
- (Red-Green-Blue) The most common color mode, which most closely resembles the sRGB profile that is standard for the majority of monitors and best describes how they generate color.
- (Cyan-Magenta-Yellow-Black) The next most common mode, it describes how colors will be composited by common printing processes.
- Intended to approximate human vision, and allows for certain edits that are useful in photo and video.
- HSB and HSL
- (Hue-Saturation-Brightness/Lightness) Two color modes based on cylindrical coordinates. The two differ in how the second and third parameters (saturation and brightness or lightness) interact. Although no longer present as color modes in current versions, HSB remains as an option in the Color Picker, Info, and Color Sampler palettes; the Hue/Saturation adjustment and the Luminosity blending mode is based on HSL.
Special color modes
- A printing-oriented mode that, contrary to its name, allows for monotones, duotones, tritones, and quadtones. Channels are not directly accessible, but are manipulated by proxy through Curves.
- A "super-mode" that allows for spot colors to be used. All other modes can be considered special cases of Multichannel mode. In this mode, Layers are disabled, as compositing is too complex to allow their use.
Changing color modes
Color modes use different values to represent similar colors. In addition, a color available in one mode might not have an equivalent in another mode.Also, some adjustments and filters are not available in certain color modes.
Because of these issues, changing an image's color mode is a serious decision.
In the case of CMYK mode—particularly limited in range—the View > Gamut Warning command shows what colors cannot be properly rendered. The image should be adjusted to eliminate these out-of-gamut colors—changing to CMYK mode will automatically map such colors to their nearest equivalent, which may adversely affect the image.