Choose the Correct Photoshop Mode: Index Color, RGB, CMYK
Choose Image→Mode to view image mode choices available in Adobe Photoshop CS5. Selecting the right one for an image is important because each mode offers different capabilities and results.
Even if you don’t work in Index color, you probably have saved a file in this mode. Indexed Color mode uses a color lookup table (CLUT) to create the image.
A CLUT contains all colors that make up an image, such as a box of crayons used to create artwork. If you have a box of only eight crayons that are used to color an image, you have a CLUT of only eight colors.
Of course, your image would look much better if you used the 64-count box of crayons with the sharpener on the back, but those additional colors increase the size of the CLUT and the file size.
The highest number of colors that can be in Index mode is 256. When saving Web images, you often have to define a color table.
Choose Image→Mode→Color Table to see the color table making up an image.
RGB (Red, Green, Blue) mode is the standard format you work in if you import images from a digital camera or scan images on a scanner. For complete access to features, RGB is probably the best color mode to work in. If you’re working on images for use on the Web, color copiers, desktop color printers, and onscreen presentations, stay in RGB mode.
If you're having an image printed on a press (for example, if you’re having it professionally printed), it must be separated. Don’t convert images to CMYK mode until you’re finished the color correction and you know that your color settings are accurate. A good print service may want the RGB file so that it can complete an accurate conversion.
CMYK (Cyan, Magenta, Yellow, Black) mode is used for final separations for the press. Use a good magnifying glass to look closely at anything printed in color and you may see the CMYK colors that created it. A typical four-color printing press has a plate for each color and runs the colors in the order of cyan, magenta, yellow, and then black.
Don’t take lightly the task of converting an image into this mode. You need to make decisions when you convert an image to CMYK, such as where to print the file and on which paper stock, so that the resulting image is the best it can be. Talk to your print provider for specifications that are important when converting to CMYK mode.