First, forget about CIE Lab color unless you already have a background in color theory. It's there. Photoshop uses it in the background, but you needn't concern yourself with it. The other three models—HSB, RGB, and CMYK—will have much greater impact on your work in Photoshop. The difference between the modes and the models is simple. The models are methods of defining color. Modes are methods of working with color based on the models. HSB is the only model without a directly corresponding mode. CMYK and RGB have corresponding modes in Photoshop. There are also modes for black-and-white, grayscale, and limited color work.
The Photoshop modes available under Image→Mode are as follows:
Bitmap
Grayscale
Duotone
Indexed Color
RGB Color
CMYK Color
Lab Color
Multichannel
There are only four of these color modes that you'll use often: Grayscale, RGB, CMYK, and Indexed Color. Let's take a closer look at them.
Let's start with the most basic of the color modes available within Photoshop—Bitmap and Grayscale.
The Grayscale mode offers 256 shades of gray that range from white to black, whereas the Bitmap mode uses only two color values to display images—black and white (see Figures 5.4 and 5.5 for examples).
Notice the vast difference in quality. The Grayscale image has a smooth transition between values, whereas the Bitmap image does not. There are, however, a number of ways to convert to Bitmap mode, discussed later in this hour.
Whenever a picture is printed in black-and-white or grayscale—for instance, as part of a newsletter or brochure—it makes sense for you to work on it in Grayscale mode. Doing the conversion yourself, rather than sending a color photo to the printer, gives you the opportunity to make sure that the picture will print properly. You can tell by looking at it whether the darks need to be lightened or the light grays intensified to bring out more detail. You can adjust the overall level of contrast as well as work on individual trouble spots.
To convert a color photo to Grayscale, simply choose Image→Mode→Grayscale. You'll be asked for permission to discard the color information. Click OK to confirm, and the picture is converted to grays. To convert the picture to Bitmap mode, as you might want to for certain effects, you must first convert to grayscale and then to bitmap.
RGB is the color mode for working on pictures that will be viewed on a computer screen. If you are preparing pictures in Photoshop that will eventually become part of a desktop presentation, a video, or a web page, stick with RGB for the best color rendition. If your work is only going on the Web, I still recommend doing the color adjustments in RGB and then converting the picture to Indexed Color if you choose to save it as a GIF in its final form. Also, if you work in Indexed Color, you can't use Photoshop's filters or layers. That's too much of a limitation!
Indexed Color, when it can work for you, is a wonderful thing. Because of cross-platform compatibility issues, web designers are theoretically limited to the 216 colors shared by Macintoshes and PCs. Indexed Color is a palette or, rather, a collection of colors—256 to be exact. With this mode, you know exactly what you are getting, and if you don't like any of the palettes Photoshop supplies, you can build your own. Many web designers stick to indexed palettes to ensure consistent color. Others use any colors they want, knowing that most users don't calibrate their monitors anyway.
Indexed Color is perfect for the World Wide Web. The Indexed Color mode includes a specific Web palette. Indexed Color doesn't really limit you to 216 colors, however. Dithering takes place in Indexed Color images. From RGB mode, choose Image→Mode→Indexed Color to take a look at the Indexed Color dialog box (see Figure 5.6).
Dithering means that certain colors are combined, that is, adjacent pixels are interspersed, visually blending onscreen to create a new color although they retain their original color—or the closest index equivalent—when viewed at a large magnification.
You are given a number of palette choices when you work with Indexed Color. They are as follows:
Exact—This option takes the colors that are in the RGB version of the image for its palette. This works only if there are fewer than 256 colors in the original image.
System (Mac OS)—This option uses the Macintosh System palette.
System (Windows)—This option uses the Windows System palette.
Web—This palette uses the 216 colors discussed previously. If you are planning to publish your work on the World Wide Web, this is the “safe” palette. Otherwise, you might have problems with incompatible colors dropping out when an image is viewed with a web browser.
Uniform—The Uniform option bases the colors in the palette on a strict sampling of colors across the color spectrum.
Perceptual—This option creates a custom palette by giving priority to colors for which the human eye has greater sensitivity. You can use a local palette (based on the current image) or, if you switch to ImageReady, a master palette that draws colors from a group of images you plan to display on a website or CD-ROM.
Selective—The Selective option creates a color table similar to the Perceptual color table, but favoring broad areas of color and the preservation of Web colors. Again, you can choose a local palette or a master palette.
Adaptive—This is your best bet for most work in Indexed Color. During conversion, this option samples the most frequently used colors from the original. Adaptive usually provides you with the closest match to the original image. This option also comes in both local and master flavors.
Custom—If none of the other options suits you, you can always build your own palette. See the Photoshop manual for instructions.
Previous—This option simply remembers and reverts to whichever option you chose last time you converted to Indexed Color.
As you saw earlier, CMYK mode should be used only when your image is printed commercially. By converting to CMYK before you start to print (and being aware of gamut warnings), you can make sure that your nice yellow banana or flower doesn't end up a muddy brown, or your bright blue sky doesn't print as purple.
Gamut refers to the range of colors that the combination of CMYK inks can print. Some colors are out of gamut, and can't be printed accurately. Very bright colors, particularly oranges and greens, are often out of gamut, and would trigger the gamut warning. The gamut warning shows up on the Color palette and looks like a small triangular traffic warning sign with an exclamation point in the middle.
All you have to do to convert between modes, at least mechanically (this is not taking image degradation or changes into account), is choose Image→Mode and then choose your poison.
Although Photoshop uses the model (Lab) with the broadest gamut of color to change color modes (as if all the other modes are circles that will fit within Lab color), this is no guarantee that your colors will turn out the same in another mode as they did in the original mode.
The rule of thumb is this, and I can't stress it enough—Do your work in RGB, even if you are going to output your images to print. Convert a copy of your image to CMYK immediately before you send it to the commercial print shop. To see whether all your colors are within the CMYK gamut, use the menu command View→Gamut Warning. If you are going to publish your images on the Web, stick with RGB or use Indexed Color, if file size is an issue. Knowing this will save you many hours of wondering why the Web page that looks great on the office Macintosh looks funky on the Windows machine you use at home, or why the yellow in your printed piece looks brownish.
Try it YourselfGetting Started with Color Just for fun, why don't we dive in with some hands-on before we go any further? Because the pictures in the book are in black-and-white, working through this exercise will give you a better idea of the concepts and ideas that we have been talking about. Let's look at a colorful image and examine how the modes affect the way the color appears.
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If you have a color printer, you might want to revert to RGB and print your picture and compare it to what you see onscreen. Does it look OK? If so, you're in luck. Your monitor is accurately calibrated. If not, you need to calibrate your monitor so that the images onscreen accurately display the colors as they print. Calibration is covered in a Note in Hour 23, “Printing and Publishing.” If your monitor seems to need calibration, you can jump ahead to “What's Color Management” on page 441.
Color Is Critical
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