Color modes

Photoshop lets you choose a color mode for each document. The color mode determines what color method is used to display and print the image you’re working on. By selecting a particular color mode, you are choosing to work with particular color model (a numerical method for describing color). 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.
Note: ImageReady only uses the RGB mode to work with images, because its documents are primarily intended for web display.

RGB Color mode
Photoshop’s RGB Color mode uses the RGB model, assigning an intensity value to each pixel. In 8-bits-per-channel images, the intensity values range from 0 (black) to 255 (white) for each of the RGB (red, green, blue) 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 neutral gray. When the values of all components
are 255, the result is pure white; when the values are 0, pure black.
RGB images use three colors, or channels, to reproduce colors on-screen. In 8-bits-per-channel images, the three channels translate to 24 (8 bits x 3 channels) bits of color information per pixel. With 24-bit images, up to 16.7 million colors can be reproduced. With 48-bit (16-bits-per-channel) and 96-bit (32-bits-per-channel) images, even more colors can be reproduced. In addition to being the default mode for new Photoshop images, the RGB model is used by computer monitors to display colors. This means that when working in color modes other than RGB, such as CMYK, Photoshop interpolates the CMYK image to RGB for display on-screen.
Although RGB is a standard color model, the exact range of colors represented can vary, depending on the application
or display device. Photoshop’s RGB Color mode varies according to the working space setting that you specify in the Color Settings dialog box.
Note: ImageReady uses only the RGB mode to work with images.

CMYK Color mode

In the 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 in RGB and then convert to CMYK at the end of your process. In RGB mode, you can use the Proof Setup commands to simulate the effects of a CMYK conversion without changing the actual image data. You can also use CMYK mode to work directly with CMYK images scanned or imported from high-end systems.
Although CMYK is a standard color model, the exact range of colors represented can vary, depending on the press and printing conditions. Photoshop’s CMYK Color mode varies according to the working space setting that you specify in the Color Settings dialog box.

Lab Color mode
The Lab Color mode has a lightness component (L) that can range from 0 to 100. In the Adobe Color Picker, the a component (green-red axis) and the b component (blue-yellow axis) can range from +127 to –128. In the Color palette, the a component and the b component can range from +127 to –128.
You can use Lab mode to work with Photo CD images, edit the luminance and the color values in an image independently,
move images between systems, and print to PostScript Level 2 and Level 3 printers. To print Lab images to other color PostScript devices, convert to CMYK first.
Lab images can be saved in Photoshop, Photoshop EPS, Large Document Format (PSB), Photoshop PDF, Photoshop Raw, TIFF, Photoshop DCS 1.0, or Photoshop DCS 2.0 formats. You can save 48-bit (16-bits-per-channel) Lab images in Photoshop, Large Document Format (PSB), Photoshop PDF, Photoshop Raw, or TIFF formats.
Note: The DCS 1.0 and DCS 2.0 formats convert the file to CMYK when opened.
Lab color is the intermediate color model Photoshop uses when converting from one color mode to another.

Bitmap mode

Bitmap mode uses one of two color values (black or white) to represent the pixels in an image. Images in Bitmap mode are called bitmapped 1-bit images because they have a bit depth of 1.

Grayscale mode

Grayscale mode uses different shades of gray in an image. In 8-bit images, there can be up to 256 shades of gray. Every pixel of a grayscale image has a brightness value ranging from 0 (black) to 255 (white). In 16 and 32-bit images, the number of shades in an image is much greater than in 8-bit images. Grayscale values can also be measured as percentages of black ink coverage (0% is equal to white, 100% to black). Images produced using black-and-white or grayscale scanners typically are displayed in Grayscale mode.
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 specify in the Color Settings dialog box.
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

Duotone mode creates monotone, duotone (two-color), tritone (three-color), and quadtone (four-color) grayscale images using one to four custom inks.

Indexed Color mode

Indexed Color mode produces 8-bit image files with 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 uses dithering to simulate the color using available colors.
Because the palette of colors is limited, indexed color can reduce file size yet maintain the visual quality needed for multimedia presentations, web pages, and the like. Limited editing is available in this mode. For extensive editing, you should convert temporarily to RGB mode. Indexed color files can be saved in Photoshop, BMP, GIF, Photoshop EPS, Large Document Format (PSB), PCX, Photoshop PDF, Photoshop Raw, Photoshop 2.0, PICT, PNG, Targa, or TIFF formats.

Multichannel mode

Multichannel mode uses 256 levels of gray in each channel. Multichannel images are useful for specialized printing. Multichannel mode images can be saved in Photoshop, Photoshop 2.0, Photoshop Raw, or Photoshop DCS 2.0 format.
These guidelines apply to converting images to Multichannel mode:
 – Color channels in the original image become spot color channels in the converted image.
 – When you convert a color image to a multichannel image, the new grayscale information is based on the color values of the pixels in each channel.
 – Converting a CMYK image to Multichannel mode creates cyan, magenta, yellow, and black spot channels.
 – Converting an RGB image to Multichannel mode creates cyan, magenta, and yellow spot channels.
 – Deleting a channel from an RGB, CMYK, or Lab image automatically converts the image to Multichannel mode.
 – To export a multichannel image, save it in Photoshop DCS 2.0 format.