When preparing your images for output, its important to take into account the output constraints. If your image will be output to a CMYK press (cyan, yellow, magenta, black), it will be constrained to the color range of the ink and paper. Gamut is the term used to describe the range in which a media can present its self. An example would be a vibrant blue. Your monitor is capable of producing some very beautiful gradations of blue, but the CMYK environment is terribly constrained, because blue can only be achieved by mixing combinations of CMYK ink, and can only exist independently when it is a Pantone color.
If your image has “out of Gamut” colors, Photoshop is capable of visually presenting all of the pixels that are “out of Gamut.” You can quickly see which pixels those are by hitting the short cut “Command+Shift+Y.” Out of Gamut pixels will appear as a solid color that you have defined in your preferences.
If you have out of gamut colors, you can either darken, lighten, adjust hue, or desaturate the colors to bring them back into gamut. You may want to do this for an image that will be output to CMYK so that you maintain a gradation of tone and color, rather than having a solid block of color that sits upon the edge of Gamut.
see the attached screen shots