Figure 6.6. The subtractive color model can also be represented by a 3D cube.

When you view color images on your display during image editing with Photoshop, the colors in the image file are always converted to additive (RGB) colors for displayregardless of the color model used to represent the actual image. Howeverand this is importantthe color model used for the actual file remains the same, unless you change modes and then save the file. In truth, Photoshop uses another color model, called Lab (discussed below), as an intermediate when converting from one color to another.

For example, if you load a file that has been saved using the CMYK color model, a program like Photoshop will let you work on it in CMYK mode, even though the colors must be converted to RGB for viewing. In general, CMYK colors will seem less saturated and duller than RGB colors. That's because RGB colors can be made brighter by pumping more light through the device you're using to view them. CMYK colors are limited by the brightness of the substrate (a "whiter" paper will reflect more light and brighter colors) and the amount of ink used.

Within Photoshop, you don't actually lose any colors unless you physically convert the file from one mode to the other. If you do convert from CMYK to RGB mode and back again, because CMYK can represent some colors that are outside the RGB gamutand vice versayou will lose some hues each time. Stick with CMYK if that's the mode your file was created in, especially if you will be outputting to a printer or color separation system that expects to work with CMYK colors. That way, you'll avoid creating RGB colors which cannot be reproduced by the CMYK output system. Photoshop will handle your file in Lab mode, and protect your colors.

Lab color was developed by the CIE as a device-independent international standard. Lab consists of three components, a luminance channel, plus a* and b* channels that represent green to magenta and blue to yellow, respectively. Because CIELab can represent all the colors found in both RGB and CMYK, it serves as a perfect intermediate format for Photoshop. There are some highly technical operations that can be carried out in Lab mode. In addition, Lab colors more closely reflect how humans perceive color, so if there is a difference in color in CIELab mode, humans will probably perceive a difference, too. That's not always the case with RGB and CMYK modes.

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