SAMS Teach Yourself Adobe® Photoshop® CS2 in 24 Hours [Electronic resources] نسخه متنی

Color Models

Figure 5.1 shows the Photoshop Color Picker. You can reach it by clicking either of the large blocks of color at the bottom of the toolbar. It has a graduated block of color, which you can click to select a particular shade, and text entry fields that display the numbers for any chosen color in each of the four color models. In addition, Photoshop gives you a Color palette, which is shown in Figure 5.2. Open it, if it's not already open, by choosing WindowColor. It has a strip along the bottom that covers the full color spectrum, plus black and white. Clicking anywhere on it sets the Color Picker to that range of colors.

Pick a Different Picker If your Color Picker doesn't look like this one, open Preferences to the General pane and reset the Color Picker to Photoshop.

RGB Model

The RGB model, which computer monitors and TV screens use for display, assigns

values on a scale of 0 to 255 for each of the three RGB primaries. As an example, pure green (as you can see in the previous figure) has red and blue values of 0, and a green value of 255. Pure white places the values of all three RGB primaries at 255. Pure black places the values of the RGB primaries at 0.

Value , in this usage, means the relative strength of the color. Because the RGB model mixes colors of light to achieve white light, the full strength is 255. When you combine all three primaries at a value of 128 (half of 255), you get medium gray.

CMYK Model

The CMYK model, used for printing, defines colors according to their percentages of cyan, magenta, yellow, and black. These are the four colors of printing inks, both in your home inkjet printer and in the fancy, high-resolution color laser printers and printing presses that service bureaus and commercial printers use. A six-color inkjet printer adds light cyan and light magenta.

Color Me Confused A thousand years ago, when I went to grade school, every child was given a box of crayons at the start of the school year. Your grade level determined the size of the crayon box. Kindergarten and first grade students received flat boxes with eight big, thick crayons per box: the primaries, the secondaries, and brown and black. That was how we learned colors, even back then. The primaries were yellow, red, and blue. The secondaries were what you got when you mixed any two primaries: orange, from yellow and red; green, from yellow and blue; and purple, from red and blue. Brown was the mix of all three primaries, and black waswell, black was black. That's how we knew them, until high school. By then, we'd outgrown crayons, of course. We filed into our physics class, and threw the previous nine years of learning out the window. The primary colors, according to the physics teacher, were red, green, and blue. And if you mixed them, you got, not brown or even the mysterious black, butwhite! Those of us still taking art classes listened to the physics lecture skeptically and went down to the art room to try mixing red, green, and blue paint. We got a sort of muddy brown, not the promised white. We asked the art teacher about this and were sent upstairs to the drama department for a demonstration. The stage lights had filters in red, green, and blue. When all the lights were on, the result was, sure enough, white light. Why? When you're dealing with light, the drama teacher explained, colors are additive. They total to white. When you're dealing with paint, you need to subtract the colors from black to get white; adding them makes that muddy brown mess. Aha! We were enlightened. When you bought your computer system, you had to deal with this issue, whether or not you knew it at the time. Your monitor uses light to produce color. That's why it's called an RGB (Red, Green, Blue) monitor. Your printer uses ink to produce color. Not red ink, green ink, and blue ink, but a set of colors called cyan, magenta, and yellow, along with the old standby black. This color system is known by its initials, CMYK. Why K for black? For a long time, K was a mystery term, but an astute reader of an earlier version of this book explained that, "K stands for Keyline, which is a thin black line printed around colors to keep them separate." Makes sense to me….

HSB Model

When artists talk about color, they generally define it by using a set of parameters called HSB. Photoshop also includes this color model. H stands for

Hue , which is the basic color from the color wheel; for example, red, blue, or yellow. It's expressed in degrees (0360°), which correspond to the positions on the color wheel of the various colors. S is

Saturation , or the strength of the color, and it's a percentage of the color minus the amount of gray in it. Pure color pigment with no gray in it is said to be 100% saturated. Neutral gray, with no color, is 0% saturated. Saturated colors are found at the edge of the color wheel, and saturation decreases as you approach the center of the wheel. If you look at the Apple Color Picker in Figure 5.3, it's a little easier to understand this.

Brightness , the relative tone or lightness of the color, is also measured as a percentage, from 0% (black) to 100% (white). Brightness is equivalent to the value used by the RGB model.

CIE Lab Model

The most encompassing of these color models is CIE Lab. It defines a color

gamut (a range of colors) that is broader than any of the other models. Because of its broad color gamut, Photoshop uses the CIE Lab model to convert from one color model to another. Lab color is defined as luminance, plus two components (a and b), which move, respectively, from green to red and from blue to yellow. Lab color is designed to be device-independent, meaning that the range of colors defined in this model isn't restricted to the range that can be printed or displayed on a particular device. However, this is probably not a model you will use frequently. Let's focus the attention now on the more commonly used models, and what you need to know to get up and running.

Choosing a Color Model I suggest that you do your color work in RGB, regardless of whether your final image will be printed or viewed onscreen as, for instance, a picture on the World Wide Web. The reason for this is simple. Even if you specify CMYK as the color model, your monitor can display only as RGB. It doesn't have cyan, magenta, or yellow pixels, except as combinations of RGB light. Rather than make it perform the conversion every time you change a piece of the image, wait and convert it when you're ready to print. Actually, you needn't ever convert to CMYK for color printing, unless you're sending the image to a commercial print shop or fine arts Iris printer. Home/office inkjets are designed to work best with RGB input.