Although Grayscale is a standard color model, the exact range of grays represented can vary, depending on the printing conditions. In Photoshop, Grayscale mode uses the range defined by the working space setting that you have specified in the Color Settings dialog box. (See “About working spaces” on page 106.)

These guidelines apply to converting images to and from Grayscale mode:

•You can convert both Bitmap-mode and color images to grayscale.

•To convert a color image to a high-quality grayscale image, Photoshop discards all color information in the original image. The gray levels (shades) of the converted pixels represent the luminosity of the original pixels.

You can mix information from the color channels to create a custom grayscale channel by using the Channel Mixer command.

•When converting from grayscale to RGB, the color values for a pixel are based on its previous gray value. A grayscale image can also be converted to a CMYK image (for creating process-color quadtones without converting to Duotone mode) or to a Lab color image.

Duotone mode

This mode creates duotone (two-color), tritone (three-color), and quadtone (four-color) grayscale images using two to four custom inks. (See “Printing duotones” on page 479.)

Indexed Color mode

This mode uses at most 256 colors. When converting to indexed color, Photoshop builds a color lookup table (CLUT), which stores and indexes the colors in the image. If a color in the original image does not appear in the table, the program chooses the closest one or simulates the color using available colors.

By limiting the palette of colors, indexed color can reduce file size while maintaining visual quality—for example, for a multimedia animation application or a Web page. Limited editing is available in this mode. For extensive editing you should convert temporarily to RGB mode. (See “Converting to indexed color (Photoshop)” on page 97.)

Multichannel mode

This mode uses 256 levels of gray in each channel. Multichannel images are useful for specialized printing.

These guidelines apply to converting images to Multichannel mode:

•Channels in the original image become spot color channels in the converted image.

•When you convert a color image to multichannel, the new grayscale information is based on the color values of the pixels in each channel.

•Converting a CMYK image to multichannel creates cyan, magenta, yellow, and black spot channels.

•Converting an RGB image to multichannel creates cyan, magenta, and yellow spot channels.

•Deleting a channel from an RGB, CMYK, or Lab image automatically converts the image to Multichannel mode. (See “About color channels” on page 93 for more information on channels.)

•To export a multichannel image, save it in Photoshop DCS 2.0 format.