When retouching photographs one should first look at white balance and contrast. White balance is normally what you ought to retouch to begin with, then contrast.The reason for correcting white balance first of all is that you can’t correct color contrast if the image has a color cast.
White balance deals with the hue or tone of the illumination within the image and normally has white as an ideal. White balance software attempts to normalize the color of the light to neutral and to do that, the software needs some whites or grays in the picture to calculate the suitable correction tint from. For the whites one can use a piece of paper or a white wall or a dedicated white card. Gray cards are manufactured for the purpose of adding a neutral gray to the photo.
White balance software comes in two varieties: automatic and manual. The manual mode usually consist of a single temperature slider for adjusting the light cool or warm. This is OK for incandescent light, but not for fluorescent light or mixed light. When converting RAW pictures, one usually has a temperature slider. Some RAW converters also have three color sliders for red, green and blue. Color sliders can somewhat correct fluorescent light and mixed light, but the problem with using color sliders is that the black and the whites usually get a bad tone. Software with an auto option usually need neutrals in the image to work well, such as a gray card or white card. Some software can dispense with that, but usually neutrals are needed.
Contrast comes in three varieties: contrast of hue, brightness and saturation. Software usually has a single slider for contrast adjustment, which addresses all three at once. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. Two sliders should be the minimum: one for luminance contrast and one for color contrast.
The standard way to manipulate contrast is simply by changing the difference between the individual R, G and B values and the middle value (128); like this: R= (R-128) * contrast + 128; and similar for green and blue. This method is only suitable for images that cover the entire brightness range. What about very dark or very pale images? In that case you can’t use 128, but have to use the average of the individual channels in the image, like this: R=(R-RAverage)*contrast+RAverage. And similarly for G and B. Using 128 attempts the same and merely assumes the picture has a full range of brightness values, in which case the average will be 128.
Another problem with contrast adjustment is that not only may the average value not be 128, but the darkest and brightest areas may not be black and white. If that is the case, one should also be able to expand the brightness range to reach black and white. This is essentially what levels adjustment does. If one’s software does not offer the option to expand brightness range, one can do it with Photoshop’s levels adjustment like this: First convert the image to Lab mode, select the L channel only and run autolevels on that. Then convert back to RGB mode.