Photoshop provides a number of choices for the color mode used to express your image. You can open files from different color modes, convert color modes in Photoshop, and save among different formats available for a given color mode.
File formats are dependent on color modes, and some format options are not available if the image data are defined in a mode not acceptable to the format. Color mode choices in Photoshop include the following:
- Bitmap. The image is expressed in two colors: black and white. In Photoshop terms, images in the bitmap mode are referred to as line art. In Acrobat terms, this color mode is called monochrome bitmap. Bitmap images are usually about one-eighth the size of a grayscale file. The bitmap format can be used with Acrobat Capture for converting the image data to rich text.
- Grayscale. This is your anchor mode in Photoshop. Grayscale is like a black-and-white photo, a halftone, or a Charlie Chaplin movie. You see grayscale images everywhere. I refer to this as an anchor mode because you can convert to any of the other modes from grayscale.
RGB files cannot be converted directly to bitmaps or duotones. You first need to convert RGB to grayscale, and then to either a bitmap or duotone. From grayscale, although the color is not regained, you can also convert back to any of the other color modes. Grayscale images significantly reduce file sizes—they’re approximately one-third the size of an RGB file, but larger than the bitmaps.
- RGB. For screen views, multimedia, and Web graphics, RGB is the most commonly used mode. It has a color gamut much larger than CMYK and is best suited for display on computer monitors.
A few printing devices can take advantage of RGB such as, film recorders, large inkjet printers, and some desktop color printers. In most cases, however, this mode is not used for printing files to commercial output devices, especially when color-separating and using high-end digital prepress.
- CMYK. The process colors of Cyan, Magenta, Yellow, and Black are used in offset printing and most commercial output devices. The color gamut is much narrower than RGB; and when you convert an image from RGB to CMYK using Photoshop’s mode conversion command, you usually see some noticeable dilution of color appearing on your monitor.
When exporting files to PDF directly from Photoshop or when opening files in other applications and then distilling them, you should always make your color conversions first in Photoshop.
- Lab. Lab color, in theory, encompasses all the color from both the RGB and CMYK color spaces. This color mode is based on a mathematical model to describe all perceptible color within the human universe. In practicality, its color space is limited to approximately 6 million colors, about 10+ million less than RGB color.
Lab color is device-independent color, which theoretically means the color is true regardless of the device on which your image is edited and printed. Lab mode is commonly preferred by high-end color editing professionals when printing color separations on PostScript Level 2 and PostScript 3 devices.
Earlier versions of PDFs saved from Lab color images had problems printing four-color separations. Now with Acrobat 6, you can print Lab images to process separations.
- Multichannel. If you convert any of the other color modes to Multichannel mode, all the individual channels used to define the image color are converted to grayscale. The resulting document is a grayscale image with multiple channels. With regard to exporting to PDF, you likely won’t use this mode.
- Duotone. The Duotone mode can actually support one of four individual color modes. Monotone is selectable from the Duotone mode, which holds a single color value in the image, like a tint. Duotone defines the image in two color values, Tritone in three, and Quadtone in four. When you export to PDF from Photoshop, all of these modes are supported.
- Indexed Color. Whereas the other color modes such as RGB, Lab, and CMYK define an image with a wide color gamut (up to millions of colors), the Indexed Color mode limits the total colors to a maximum of 256. Color reduction in images is ideal for Web graphics where the fewer colors significantly reduce the file sizes. You can export indexed color images directly to PDF format from Photoshop.