Color Models, Spaces, And Modes; Adjusting Color Hue, Saturation, And Brightness - Adobe Photoshop CS6 User Manual

Color wheel
R. Red Y. Yellow G. Green C. Cyan B. Blue M. Magenta
For example, you can decrease the amount of any color in an image by increasing the amount of its opposite on the color wheel—and vice versa.
Colors that lie opposite each other on the standard color wheel are known as complementary colors. Similarly, you can increase and decrease a
color by adjusting the two adjacent colors on the wheel, or even by adjusting the two colors adjacent to its opposite.
In a CMYK image, you can decrease magenta either by decreasing the amount of magenta or by increasing its complement, which is green (the
color on the opposite side of the color wheel from magenta). In an RGB image, you can decrease magenta by removing red and blue or by adding
green. All of these adjustments result in an overall color balance containing less magenta.

Color models, spaces, and modes

A color model describes the colors we see and work with in digital images. Each color model, such as RGB, CMYK, or HSB, represents a different
method (usually numeric) for describing color.
A color space is a variant of a color model and has a specific gamut (range) of colors. For example, within the RGB color model are a number of
color spaces: Adobe RGB, sRGB, ProPhoto RGB, and so on.
Each device, like your monitor or printer, has its own color space and can only reproduce colors in its gamut. When an image moves from one
device to another, image colors may change because each device interprets the RGB or CMYK values according to its own color space. You can
use color management when moving images to ensure that most colors are the same or similar enough so they appear consistent. See
colors sometimes don't match.
In Photoshop, a document's color mode determines which color model is used to display and print the image you're working on. Photoshop bases
its color modes on the color models that are useful for images used in publishing. You can choose from RGB (Red, Green, Blue), CMYK (Cyan,
Magenta, Yellow, Black), Lab Color (based on CIE L* a* b*), and Grayscale. Photoshop also includes modes for specialized color output such as
Indexed Color and Duotone. Color modes determine the number of colors, the number of channels, and the file size of an image. Choosing a color
mode also determines which tools and file formats are available. See Color modes.
When you work with the colors in an image, you are adjusting numerical values in the file. It's easy to think of a number as a color, but these
numerical values are not absolute colors in themselves—they only have a color meaning within the color space of the device that is producing the
color.

Adjusting color hue, saturation, and brightness

Based on the human perception of color, the HSB model describes three fundamental characteristics of color:
Hue Color reflected from or transmitted through an object. It is measured as a location on the standard color wheel, expressed as a degree
between 0° and 360°. In common use, hue is identified by the name of the color, such as red, orange, or green.
Saturation Strength or purity of the color (sometimes called chroma). Saturation represents the amount of gray in proportion to the hue, measured
as a percentage from 0% (gray) to 100% (fully saturated). On the standard color wheel, saturation increases from the center to the edge.
Brightness Relative lightness or darkness of the color, usually measured as a percentage from 0% (black) to 100% (white).
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