A Good Beginning
The default Image Mode in Photoshop for photographic images is RGB. It is the Mode in which you work for output to a connected printer. RGB Mode is 8 bits per color channel, or 24-bit color depth, that has the maximum potential of reproducing 1.7 million different RGB value combinations, or that many distinct colors. Today, many capture devices and image sources function at a greater color depth of 10, 12, 14, or 16 bits, actually recording many more different colors than 1.7 million.

So, considering the fact some of that information captured is irrelevant to the subject and will be eliminated as part of the color correction process, and that the capture itself may have a smaller dynamic range than the color space (gamut), it makes sense to color correct at the bit depth of the original capture device, if possible. In other words, the more information you have to begin with, the more that will be there at the end once color correction is completed. Unfortunately, only the full versions of Photoshop supports opening and editing images at greater than 24-bit RGB color depth. However, if you have the option of outputting from a scanner or digital camera at the device's full bit depth and saving in a non-compressed format like TIFF, and then using the full version of Photoshop for color correction, you stand a better chance of obtaining finished 24-bit RGB files with more color information preserved.

Photoshop's Color Correction Tools
The Photoshop tools you will need and those I recommend are only four:

1. Levels
2. Curves
3. Color Balance
4. Hue/Saturation

The one overall, fundamental rule governing the use of the four tools is that they only be used in the order they appear above. The reason is very simple and functional, and that is: if you attempt to make a Color Balance or Hue/Saturation change and then follow with an adjustment of either Levels or Curves, the adjustments made with Levels or Curves will also change the values of the color and you will have to redo Color Balance and Hue/Saturation.

Photoshop Elements users should, if they have not already discovered it, know that Adobe has not included Curves in that application. So, to adjust an image's overall contrast in Elements I would suggest using the contrast adjustments available in the Color Variations tool by Lightening or Darkening highlights, midtones, and shadows selectively. The Color Variations dialog provides the distinct advantage of selecting and then making adjustments to the image on the basis of a side by side before-and-after pair of large thumbnail windows. After using Color Variations to adjust image contrast to provide good detail in tones throughout the range of the photo, you will need to use Levels (again) to be sure the gamut is optimized.

One of the utilities that is extremely valuable in all versions of Photoshop that's accessible from the Edit menu is the Histogram. This graph provides an indication of how the space in an image file is used by the information contained. If there is any blank space on either side of the black graph area that represents image data, then the space (gamut) is not optimally utilized. This indicates you should use the Levels tool to spread the available data out to fill the space (gamut). The Histogram is also useful after you have made an adjustment with a tool like Color Variations to check if its use has either compressed the image information or
pushed some of the information out of the gamut space, which is referred toas clipping, and which should be avoided at all costs to preserve highlight and shadow detail information.

Using Photoshop Levels To Easily Optimize An Image
The first thing to do after opening a new photo image in Photoshop is to go to the Image menu, then Adjustments, and click on Levels. If in the Levels dialog the graph has space at either end, then use your mouse cursor to move the triangle under the graph in to the point the graph indicates as the beginning of image information. Then, if the image appears too light or dark on screen, move the center triangle to set the midpoint farther left if the image is too dark, or farther right if too light. Then Click OK and the image information will be expanded to fill the color space (gamut).

Using Levels To Set White/Black And Gray And Remove Color Casts
Many of the images acquired from scans of film and prints as well as digital camera files often contain color casts, colors which are from a source other than the colors of the subject of the photograph. These color cast sources may include lighting sources like open sky or the light reflected from the environment that introduces color to a subject's reflected image. In scans color shifts may be caused by processing anomalies, the film base or paper base in a print, as well as color shifts due to age and decomposition of image dye colors.

Setting White, Black, And Middle Gray
Color casts can be removed as part of image optimization using the Eyedropper mouse cursor tools provided in the lower right corner of the Levels dialog. Here is where we set the White Point. After opening a new image in Photoshop use the Navigator to zoom in to the portion of the image with the brightest highlights so you can see the individual pixels. Also, go to the Window menu and open the Info dialog so the Info window is on screen. Then, open the Levels dialog and click on the far right Eyedropper icon button. Move the mouse cursor over the pixels that make up the brightest highlights and read the RGB values. The brightest value has the highest numeric values and if it contains a color cast the R, G, and B values are different. When you have identified the brightest pixel you want to set as the White Point of the image click on it.

Next, using the Navigator, move the enlarged view to the part of the image which has the darkest shadows in the picture. Then click on the far left Eyedropper icon button, and move the mouse cursor to the pixels that look the blackest. Again, using the numeric RGB values read-out in the Info window, find the one pixel with the lowest values--note if the values for each RGB channel are different, and if so, that identifies a color cast. With the darkest, lowest value pixel identified, click on it and the black point will be set and the color cast removed.

If there is an area within a photo image which should be neutral gray, then the middle Eyedropper icon button can be clicked, and, again, using a magnified view of that area that should be gray, a pixel can be selected and clicked on to shift the image values to neutral, thus removing the color cast.

In the example shown of a brick wall the scene was illuminated by an open sky that added a blue cast to the photo, which was removed by setting the gray point. There is a danger associated with setting the gray point, and that is if the image does not have a mid-tone color cast all of the values in the picture can be skewed. This is caused by misidentifying an area in the image that "should" be gray. So, it should not be assumed that an image must have the gray point set. Even so, scanned images in particular may result in cleaner, brighter color by setting the white and black points and removing any color cast exhibited at the two ends of the range of brightness values.

Straight Talk About Using Curves
The function of Photoshop's Curves tool is to change the relationship of levels of image brightness and darkness relative to each other. When you open the Curves dialog the image is represented by a straight diagonal line from the darkest tone at the lower left corner to the lightest at the opposite corner. (Note: With a gray scale image the Curves dialog has inverted values with the lightest at the bottom left, and the darkest at the top right corner.) By clicking the mouse cursor on any value within the open image on screen a "blip" will appear on the curve line indicating where on that line the value you clicked is located. If you use the mouse cursor to click at that point on the curve line a dark node will appear. The node locks that point in place unless you click on it, then hold the mouse button down and move the node--the line then curves in response to the movement and that part of the image will become lighter or darker depending on whether you move the node upward and to the left to lighten, or darker if moved downward and to the right.

To use Curves to increase contrast, first set a node at the center of the curve line to lock in the mid-tone values, and then set a node point at the center of the lower quadrant as well as at the center of the upper quadrant. Then move the upper node up and to the left a little, which will lighten the highlights of the image. Go to the bottom node and move it down and to the right a little, which will darken the shadow values. The result is the image appears to have more contrast as well as color intensity.

If you have an image which is too contrasty, like a portrait that appears too harsh with poorly defined detail in highlights and shadows, start with the same setting of three nodes. Then, to reduce contrast reverse the "S" curve by lowering and moving the upper node to the right, as well as moving the lower node up and to the left.
Photoshop's Curves tool can be used to correct very specific image problems as well as just increase or decrease contrast. In the example of the shot of a rail-yard shop, the shadow in the foreground was originally very dark, largely obscuring the detail of the tracks converging toward the shop building. By setting the nodes at the mid point and at the intersection of the top quadrant, all the mid to lighter values were locked in place. Then, by setting a node close to the bottom of the curve line and moving it upward the foreground was lightened opening up the detail in the shadow. An in-between node was also set and brought down a bit to limit
the lightening to just the darkest values in the shadows.

When To Use Photoshop's Color Balance Tool
Photoshop's Color Balance tool has its uses but they are limited by two factors. Unlike many scanning software color balance tools, the Photoshop tool only functions on separate Shadows, Midtones, and Highlights portions of the image--there is no global means to adjust the overall color balance. In addition, the tool relies on separate RGB sliders that demand the user have an understanding of how color is structured, rather than a more intuitive graphic single tool like a color wheel.

The Photoshop Color Balance tool does have advantages, however. In the original portrait image shown, the subject is illuminated naturally by light from two windows. The image had a color shift from the one window to the far right, which resulted in greenish highlights. Using the Color Balance tool and setting it on Highlights, I moved the Cyan/Red slider to the Red side until the greenish shift disappeared from the highlights and the complexion tones.

Using Photoshop's Hue/Saturation Tool
Of all the tools to adjust color, Photoshop's Hue/Saturation tool is both the most powerful and the most complex. Its effective use demands as much exploration and practice as you can afford. When you activate Hue/Saturation the dialog provides first of all the Edit choice, with the default being "Master." If chosen it applies changes globally to all colors in the image. At the bottom are two color bars that represent the color spectrum. One bar is static and the other is dynamic to display any changes input with the three sliders in the Edit section of the dialog.

The Hue slider changes the relative relationship of the two color bars right to left. If you move the Hue slider to the right a red will become more yellow in content, or to the left it will become a bluish red. The Saturation slider moved to the right increases the intensity of color and to the left reduces the saturation. The bottom slider, Lightness, does pretty much what it says, lightens and darkens, and is only really useful when applied to a selected color. At the top again, rather than Master you can choose any of six bands of color, which, when selected, activates the appearance of "brackets" between the two color bars and facilitates selective color adjustments limited to just one color.

The range of the color band selected may be narrowed or expanded by using the mouse cursor to move the brackets manually. The center, darker section, is the band that receives full adjustment and the areas in brackets on each side are a feathered space of gradually decreasing effect.

The portrait example was chosen because in people close-ups there very often is not a pure white or black in the subject, and using the Set Black/White Point results in an image that is too contrasty and harsh. So in this example I used the easy Levels adjustment, leaving just a little fudge space on the two ends of the Histogram to preserve the soft tones of the image. However due to age of this scanned slide a yellow coloration had occurred. So I used Hue/Saturation set on the Edit channel Yellows, and then clicked on the yellow in the background with the mouse cursor to adjust the position of the brackets to the actual tint of yellow in the image.
I then moved the Saturation slider to the left, reducing the yellow saturation until the tone values appeared gray. Sometimes if you use this method to remove a strong color shift in an image, the adjacent colors will also lose too much saturation. Then, move the reduction of the selected color even 10 or 20 points farther, and switch the Edit mode to Master and increase Saturation by moving the slider to the right until the adjacent colors have been restored in intensity.

Multiple Selective Color Adjustments
A common source of excess blue in a slide is when the primary light source is open sky. Hue/Saturation was used to correct this scene of a storm moving in from the west about midday. I set Edit to Blues, clicked on the clouds and reduced the saturation drastically. Although the clouds and much of the image then looked as I remembered, the wheat field was too rusty colored, so I set Edit to Red, and clicked on the wheat in the field. (Note: If the color is on the cusp, Photoshop will arbitrarily rename the chosen color. In this case, "red" has been switched to "yellow.") I then moved the Hue slider to the right to make the color more yellow and boosted the saturation resulting in a quite natural golden-looking rendition of the wheat field in the scene.