eSchool - Digital Photography, May 2nd, 2007
Color correction is a powerful retouching tool. To use it effectively, though, you should understand some basics about color in Photoshop CS2 (version 9.0) for Windows.
Explore the Color Picker
To begin, open the Color Picker in Photoshop by double-clicking the foreground color selection box on the Toolbox. Notice the four areas on the lower right with a character and values, plus the large color field on the left.
These represent different color modes that Photoshop supports that are based on color models. A color model is simply another way to describe color with a numerical value. The four color modes in the Color Picker are RGB, HSB, CMYK, and LAB. The letters for the first three modes stand for red, green, blue (RGB); cyan, magenta, yellow, black (CMYK); and hue, saturation, brightness (HSB).
Note: LAB color mode is not covered in this article. For further information on the LAB color model, refer to Photoshop Help.
All digital images start out in RGB color mode because of the process digital cameras, scanners, and digital video use to convert light into its RGB components. Therefore, you must convert an RGB image to another color mode for different types of final output.
The CYMK and LAB color modes are used mainly for printing images for publication on high-end PostScript printers and presses. The four-color printing process creates the image with dots of the four ink colors -- CMYK. It can also use special premixed inks. These standardized premixed inks in the United States are referred to as the Pantone Matching System (PMS). Consult with your printer about the color mode requirements for a specific print job. If the printer requires your images to be in CMYK color mode, it's easy to do in Photoshop.
To convert an RGB image to CMYK:
- Open any digital image. Notice that RGB appears in the title bar.
- Select Image > Mode > CMYK Color.
That's how easy it is to convert from RGB to CMYK. You may notice a difference in the color as Photoshop simulates the printed ink colors on the RGB monitor.
Additive Color vs. Subtractive Color
The color for the images displayed on the monitor is produced with different types of light projection, and the color for your images printed on paper is produced with printing inks. This division of color is referred to as the Light Theory of additive color and the Pigment Theory of subtractive color.
RGB color mode is based on the additive color model. Equal portions of red, green, and blue are added to become white. The opposite happens in CMYK mode, which is based on the subtractive color model. In this model, when you add all of the colors, the result is black.
The range of colors in RGB color mode is based on the spectrum of visible light. The color slider bar in the Color Picker dialog box represents the color spectrum. If you bend the color spectrum into a circle, you'll have the Color Wheel.
Notice the positions of the colors on the Color Wheel. This makes it easy to see the relationships of the colors and the color opposites. For example, cyan is opposite red, and cyan is made from the two adjacent primary colors of blue and green. Remember that this is the additive color model, which is different from the pigment (subtractive) model.
To see how this relates to Photoshop:
- Open any of your images.
- Select Image > Adjustments > Color Balance. In the Color Balance dialog box, you can see the correlation of the color opposites.
As you recall, a color model is simply another way of describing color with a numerical value. This way of expressing color allows the computer to work with the color data. Each pixel on the monitor has a value for each of the three colors of RGB. The numerical range is from 0 to 255. In the additive color model, black represents the absence of color, so the value for black is 0. White represents all of the colors, so the value for white is 255. Fifty percent of all of the colors is a medium gray.
The RGB values for a color are usually written with the characters RGB and the value. A color value in RGB mode is written with the three values for the color. For example, the value for cyan is RGB (0, 255, 255), as shown in the Color palette of Photoshop. You can view the same information in the RGB section of the Color Picker.
The other color models that are relevant to color adjustments in Photoshop are HSB and HSL. The HSB model represents the colors based on hue, saturation, and brightness. The HSL model is a variation of the HSB model and is based on hue, saturation, and lightness. The difference between the two is that the lightness is based on linear changes of how light or dark the color is and the brightness of the HSB model is based on non-linear changes. In Photoshop, you'll see the HSB color model in the Color Picker.
Note: Hue is defined as the color's name, such as red; saturation measures how vivid or pure the color is; brightness measures how light or bright it is.
To see how the color field changes in the Color Picker when you select each one of the elements of the HSB color model, do the following:
- Open any image.
- Use the Eyedropper tool to sample a color.
- Double-click the foreground color selection box to open the Color Picker.
- Select the Saturation option (labeled S). The color field shows all the hues with the maximum saturation at the top and the minimum at the bottom.
- Select the Brightness option (labeled B). The color field shows all the hues with the maximum brightness at the top and the minimum at the bottom.
- Close the Color Picker.
With this basic knowledge about color and color theory, you're ready to experiment with creating custom color effects in your own digital images.
