When you work with Photoshop, it lets you view and adjust your colors in all sorts of different ways. It's difficult to adjust saturation in an image, for example, by manipulating RGB or CMYK values directly, so Photoshop provides tools that let you apply changes in hue, saturation, and brightness to the underlying RGB or CMYK data. Likewise, if you have an RGB image but you plan to print it in CMYK, you can use Photoshop's Info palette to keep track of what's happening to the CMYK values you'll eventually get when you do the mode change from RGB to CMYK.
You face two fairly large problems, however. The first is that every time you do a mode change, you lose some image information; if your images start with 256 shades of each color, some of these get lost due to rounding errors as you convert from one color space to another. Photoshop lets you work around this by providing information about what you'll get after you've done the color-space conversion, without your actually having to do so until we've perfected your images. We discuss this in much more detail in Chapter 6, Image Adjustment Fundamentals.
The second problem is that the color spaces in which most of your images are stored, RGB and CMYK, are device dependentthe color you'll get varies depending on the device you send it to. Worse, each device has a range of colors it can reproducecalled the color gamutand some devices have a much wider gamut than others (see Figure 4-4, earlier in this chapter). For example, color film can record a wider range of colors than a color monitor can display, and the monitor displays a wider range of colors than you can reproduce with ink on paper; so no matter what you do, some of the colors captured on film simply can't be reproduced in print.
Fortunately, Photoshop has tools that let us remove some (if not all) of the ambiguity from your RGB and CMYK color definitions, and it lets you specify the gamut of your monitor and your CMYK output devices. The next chapter, Color Settings, is devoted to explaining what those tools are, how they work, and how to use them.