Photoshop’s RGB color mode uses the RGB model, assigning an intensity value to each pixel ranging 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 value of all components is 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. 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 images (16 bits per channel), 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 with 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 have specified in the color settings dialog box.
In Photoshop’s 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, while the darker (shadow) colors are assigned 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 color 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 (see soft-proofing colors). 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 have specified in the color settings dialog box.
In Photoshop, 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 +120 to -120.
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 to 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), PDF, Photoshop Raw, TIFF, Photosohp DCS 1.0, or Photoshop DCS 2.0 formats. 48-bit (16 bits per channel) LAB images can be saved in 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.
This 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.
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.
By limiting the palette of colors, indexed color can reduce file size while maintaining enough visual quality for certain applications, like a multimedia presentation or a web page. 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, PSB, PCX, Photoshop PDF, Photoshop Raw, Photoshop 2.0, PICT, PNG, TARGA, or TIFF formats.
Duotone creates monotone, duotone (two-color), tritone (three-color), and quadtone (four-color) grayscale images using two to four custom inks.
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 to multichannel mode:
- Color channels in the original 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
- To export a multichannel image, save it in Photoshop DCS 2.0 format