Here's a sample writeup from the 50+ page classroom Photoshop text. Note that this text is designed to be used in conjunction with class instruction rather than as a stand-alone text. It is likewise designed to minimize the amount of note-taking required during the session, and also to be used for personal study and reference after the workshop has ended.
Precise color balancing often requires a combination of controls. Individual adjustment of channels within Curves is usually the best starting place; additional adjustments to Hue/Saturation and sometimes Selective Color are often necessary. “Color Balance” is a less refined adjustment and is best avoided in favor of these superior controls. Of course, each of these adjustments should be applied on its own Adjustment Layer. (Adjustments to individual color channels in Curves can be done on the same Curves layer as an RGB tonality adjustment.)
Identifying and removing a color cast is a most important aspect of color balancing. While certain color casts are often considered pleasing, such as a warm cast caused by sunlight near sunrise or sunset, the blue cast of cloudy or overcast light is less pleasing and should usually be removed or lessened for attractive results. Similarly, a magenta or cyan cast is often introduced either through the scanning process or sometimes by film itself, and must also be removed through color correction. Observe areas that should be a known color, particularly neutrals such as white, grey bark or rock, etc. If these neutrals exhibit an inappropriate cast, correcting them will usually bring the rest of the image in line. I’ve found it very helpful to imagine side-by-side sheets of plain white and black paper in the image; this seems to counteract the eye’s tendency to provide “automatic white balance” to whites that aren’t really white. Photoshop’s color-balancing eyedropper controls sometimes help, but as often as not they don’t; try them for a start, then tweak the result if it goes in the right direction, or undo if it doesn’t. Look at each color channel’s individual curves after trying the eyedropper controls to see how the curves have been altered.
Examining RGB values in the Info palette is invaluable in correcting color casts. Observe areas that should be neutral, such as white highlights and grey rock, bark, etc. Color casts in these areas are readily visible to the eye with practice, and appear in the Info palette as differing numbers in the three color channels. For example, let’s say that you identify a cast in what should be a neutral grey rock. Looking at the Info palette, its RGB values are 125, 118, and 125 respectively. A perfect neutral grey would theoretically read 125/125/125. If the middle number (Green channel) were higher than 125, this would indicate a green cast. Keeping in mind that the opposite of green is magenta, the green channel’s lower reading indicates a magenta cast.
Let’s take another example: bright white birch trunks. White highlights photographed in overcast light will usually appear blue. While you may want some blue in snow for a natural look, in many subjects a clean white results in a more pleasing appearance. Observe the brightest areas of trunk that would look their best if they are a clean white. Let’s say that these bright areas read 225/230/240 in R,G,B channels respectively. Take the number that is numerically in the middle of the three values (here, 230 in Green channel) as your base figure and compare the other channels. Red channel at 225, in relation to “base” Green channel at 230, indicates 5 points of Cyan cast; Blue channel at 240, in relation to “base” Green channel at 230, indicates 10 points of Blue cast. You now know that you need to adjust both Red and Blue channels to produce neutral white birch trunks.
Hue/Saturation (see below) is also useful in identifying a color cast. Temporarily increase saturation by a substantial amount, causing subtle color shifts to become pronounced and easily visible; the errant color can then be removed or altered by one of the following methods.
To manually adjust color, use Curves in each color channel and begin by adjusting the channel in most need of correction. Within each channel, adjust midtone first, then tweak the curve if required. Color balancing is a delicate process and often requires several iterations, making rough corrections to each channel as needed, then going through the channels again for fine-tuning. As proper color balance is approached, a little correction goes a long way; a point or two in either direction can make a noticeable difference.
Remember that reducing one color via Curves will increase its opposite, meaning that an opposite color cast can easily be created if one applies too much correction. The components of each channel include Red/Cyan, Green/Magenta, and Blue/Yellow. In some images, it’s difficult to find a happy medium (i.e. removing a blue cast turns an image too yellow); in these cases, get the image looking as good as possible and move on to the next step. It can also be helpful to stop work on an image for awhile and come back later with a fresh look.
A Hue/Saturation Adjustment Layer is usually the next step in color correction. Most images will require saturation adjustment, overall and/or restricted to particular colors. However, Hue/Saturation is useful to balance color as well. Note the “Edit” box that applies by default to the entire range of colors; below “Master” is a separate control for each of the six primary and secondary color ranges. Note the color bars beneath the Hue/Saturation sliders; if the entire image seems imbalanced toward a color near the preferred color (for example, the overall image looks too cyan instead of green), try shifting Hue’s slider arrow in the direction from cyan to green, i.e. to the left. Don’t try to shift through a broad range of colors, i.e. from cyan to orange. If only a limited color range needs to be adjusted (for example, green leaves look too cyan), change the “Edit” box’s color control to “Cyan”, then shift hue toward green. This will affect only those colors that Photoshop determines to be in the “Cyan” range. The exact color range to be affected can be adjusted via the slider controls between the two color range bars: colors between the two vertical bars are affected 100%, while colors between vertical bars and triangles are affected in a feathered manner to provide a smooth transition.
Occasionally, a stubborn color cast (blue, in particular, on images shot in very blue light) refuses to be corrected adequately through the above methods. In this instance, try reducing saturation of that particular color in Hue/Saturations Saturation control. While this will indeed reduce the offending color, be aware that it may eliminate color to the degree that certain areas of the image will be overly desaturated, i.e. approaching a black-and-white image.
Another effective method to correct color balance is with a Selective Color Adjustment Layer. It can fine-tune color to a very fine degree, since it allows alteration of individual color components within each primary and secondary color, plus those of white, neutral, and black. This control is the best way to neutralize color casts that persist in those neutral colors; note that even a very slight adjustment within the neutrals will have a substantial effect. Choose the basic color to be adjusted, then move slider arrows to bring about correction as desired. Choosing the “absolute” rather than “relative” button (below sliders) creates a more substantial amount of correction for each percentage of alteration.
As always, it may be beneficial to restrict any or all of these color-correction methods to a particular area of an image via masking.