HSB model (Hue, Saturation, Brightness)

Based on the human perception of color, the HSB model describes three fundamental characteristics of color:

• Hue is the 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, sometimes called chroma, is the strength or purity of the color. 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 is the relative lightness or darkness of the color, usually measured as a percentage from 0% (black) to 100% (white).

Although you can use the HSB model in Photoshop to define a color in the Color palette or Color Picker dialog box, there is no HSB mode available for creating and editing images.

RGB model (red, green, blue)

A large percentage of the visible spectrum can be represented by mixing red, green, and blue (RGB) colored light in various proportions and intensities. Where the colors overlap, they create cyan, magenta, and yellow. Because the RGB colors combine to create white, they are also called additive colors. Adding all colors together creates white—that is, all light is reflected back to the eye. Additive colors are used for lighting, video, and monitors. Your monitor, for example, creates color by emitting light through red, green, and blue phosphors.

The RGB model assigns an intensity value to each pixel ranging from 0 (black) to 255 (white) for each of the RGB components in a color image.

For example, a bright red color might have an R value of 246, a G value of 20, and a B value of 50. When the values of all three components are equal, the result is a shade of gray. When the value of all components is 255, the result is pure white; when the value is 0, pure black.

RGB images use three colors to reproduce up to 16.7 million colors on-screen. RGB images are three-channel images, so they contain 24 (8 x 3) bits per pixel. RGB is the default mode for new Photoshop images.

Computer monitors always display colors using the RGB model. This means that when working in color modes other than RGB, such as CMYK, Photoshop temporarily converts data into RGB data for display on-screen.

CMYK model (cyan, magenta, yellow, black)

The CMYK model is based on the light-absorbing quality of ink printed on paper. As white light strikes translucent inks, part of the spectrum is absorbed and part is reflected back to your eyes. In theory, pure cyan (C), magenta (M), and yellow (Y) pigments should combine to absorb all color and produce black. For this reason these colors are called subtractive colors. Because all printing inks contain some impurities, these three inks actually produce a muddy brown and must be combined with black (K) ink to produce a true black. (K is used instead of B to avoid confusion with blue.) Combining these inks to reproduce color is called four-color process printing. The subtractive (CMY) and additive (RGB) colors are complementary colors. Each pair of subtractive colors creates an additive color, and vice versa.

In CMYK mode each pixel is assigned a percentage value for each of the process inks. The lightest (highlight) colors are assigned small percentages of process ink colors, the darker (shadow) colors higher percentages. For example, a bright red might contain 2% cyan, 93% magenta, 90% yellow, and 0% black. In CMYK images, pure white is generated when all four components have values of 0%.

Use the CMYK mode when preparing an image to be printed using process colors. Converting an RGB image into CMYK creates a color separation. If you start with an RGB image, it’s best to edit first and then convert to CMYK. In RGB mode, you can use the CMYK Preview command to simulate the effects of the change without actually changing image data (see Previewing CMYK colors). You can also use CMYK mode to work directly with CMYK images scanned or imported from high-end systems.

CMYK images consist of the four colors used to print color separations. They are four-channel images, containing 32 (8 x 4) bits per pixel.

L*a*b model

The L*a*b color model is based on the model proposed by the Commission Internationale d’Eclairage (CIE) in 1931 as an international standard for color measurement. In 1976, this model was refined and named CIE L*a*b.

L*a*b color is designed to be device independent; creating consistent color whatever the device (such as monitor, printer, computer, or scanner) used to create or output the image.

L*a*b color consists of a luminance or lightness component (L) and two chromatic components: the a component (from green to red) and the b component (from blue to yellow).