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Device-Independent Color

Basically, the problem with HSB, RGB, and CMY (and even CMYK) is that they don't describe how a color looks; they only describe the color's ingredients. You've probably walked into a television store and seen about a hundred televisions on the wall, each of them receiving the same color information. But none of them displays the colors in the same way.

Color Relationships at a Glance

It's worth spending however long it takes to understand the color relationships we're discussing in this chapter. We all have a favorite color space, but if you can learn to view color in more than one way—understanding how to achieve the same results by manipulating CMY, RGB, and HSB—you'll find the world of color correction much less alien, and you'll be much more able to select the right tool for the job.

Saturated Primaries—CMY versus RGB

The colors at left are fully saturated—each contains 100 percent of one or two primaries. The additive and subtractive primaries have an inverse relationship.

We suggest memorizing these fundamentals.

  • 100% cyan = 0 red

  • 100% magenta = 0 green

  • 100% yellow = 0 blue

  • Increasing RGB values corresponds exactly to reducing CMY values, and vice versa.

  • Reducing saturation (making something more gray) means introducing the complementary color; to desaturate red, for example, you add cyan.

  • The complement of a primary color is produced by combining equal amounts of the other two primary colors.

  • Lightening or darkening a saturated color desaturates that color.

  • Changing the hue of a color often changes lightness as well.

  • Saturation changes can cause hue changes.

Desaturating Saturated Colors

Desaturating the reds removes red and adds other primaries to the red areas. Adding a third primary “pollutes” the saturated color, causing it to go gray.It may or may not affect lightness.

Lightness and Saturation

Lightening or darkening a saturated color (here, +50 and -50) desaturates it; either it pulls the primaries back from 100 percent, or it pollutes them with a third primary, or both. Also note the hue shift in the darkened version.

In fact, if you send the same RGB values to ten different monitors, or the same CMYK values to ten different presses, you'll end up with ten different colors (see Figure 4-4). We call RGB and CMYK device dependent, because the color you get varies from device to device.

Figure 4-4. Device-dependent color and color gamuts

Since this figure is printed with process inks on paper, it can only simulate the results of sending the same RGB or CMYK values to various devices. It depicts relative appearances, not actual results. Likewise, the color chart just represents the gamuts of different devices, rather than actually showing those gamuts. Screen display

Dye-sublimation printer

Process inks, coated stock

Process inks, newsprint

Color gamuts

So, Photoshop has a problem in trying to display colors properly on your monitor: It doesn't know what the colors should look like to you. It doesn't know what those RGB or CMYK values really mean.

Plus, the program has to take all the little quirks of human vision into account. For instance, our eyes are more sensitive to some colors and brightness levels than to others, and we're more sensitive to small changes in bright colors than we are to small changes in dark ones (if you've had trouble teasing all the subtle shadow details out of your scanned images, this is one reason why). RGB and CMYK don't give Photoshop the information it needs to know what color is actually being described.

Lab Color

Fortunately, there's CIE Lab, which appears on the Mode menu simply as Lab. Lab is designed to describe what colors look like, regardless of the device they're displayed on, so we call it device independent.

Whereas in HSB the hues are represented as lying around a wheel, Lab color uses a more accurate but significantly less intuitive arrangement. In Lab, the third axis (which lies perpendicular to the page and is roughly equivalent to brightness in HSB) is the luminance axis—it represents how bright the color appears to the human eye. But unlike brightness in HSB, it takes into account the fact that we see green as brighter than blue.

Whole books have been written on Lab color (we've even read some of them), and while they may be of great interest to color scientists, they're unlikely to help you get great-looking images on a deadline. For now, there are really only three things you need to know about Lab color.

  • While HSB, HSL, and LCH are based on the way we think about color, and RGB and CMYK are based on the ways devices such as monitors and printers produce color, Lab is based on the way humans actually see color. A Lab specification describes the color that most people will see when they look at an object under specified lighting conditions.

  • Photoshop thinks in Lab when it does mode changes. For instance, when you switch from RGB mode to CMYK mode, Photoshop uses Lab to decide what color is being specified by each device-dependent RGB value, and then comes up with the right device-dependent CMYK equivalent. You'll see why this is so important in the next chapter, Color Settings.

  • Finally, you shouldn't feel dumb if you find it hard to get your head around Lab color. It is difficult to visualize, because it's an abstract mathematical construct—it isn't based on amounts of things we can understand readily, like RGB or HSB. It uses differing amounts of three primaries to specify colors, but those primaries don't really correspond to anything we can actually experience.

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