White balance adjustment means to adjust the illumination in a picture to white. To accomplish this you must have a neutral in the image like a gray card or a white wall. Software can correct white balance, but not all software solutions are equally good.
The first thing you want to take care of, when you begin to edit your pictures, is white balance. White balance refers to the color of the light and assumes that the best light color is white. Some shots, like sunset or candlelight, do not have white light, but in general an impression of white light is desirable. Some use Photoshop\’s auto levels to set white balance, but that is not ideal, since auto levels just sets the darkest pixels to black and the brightest to white without looking at the mid tones. But what if the lightest pixel in your photo is not white? Or what if you do not have pure black in the photo? (Most images have black areas, but the palest pixels are rarely pure white).
When correcting white balance, the mid tones are the most important and to help set the mid tones correctly one adds a grey card to the photo when taking the picture. A grey card is a sheet of cardboard or plastic colored an exact mid tone neutral gray. Ideally one has three cards: a black, a gray and a white. Photoshop\’s levels adjustment panel has three eye droppers for picking color: one for white, one for gray and one for black. By clicking the gray color picker on the gray card, one can adjust the mid tones to neutral gray. One can only include a gray card in the picture if one intends later to crop the image.
If one does not want a gray card in the image, or if one doesn\’t have a gray card at hand, one can later use specialized software that scans the image and calculates the color of the light and sets it to white. There are problems with such applications: what if there are no neutral areas in the picture to calculate the color of the light from? Some applications do not need a neutral in the photo, but most do to get a good result.
If you work with RAW images, you will have discovered that the RAW converters generally come with a control for color temperature, which means a control to adjust the photo cool or warm. But what if the color of the light is greenish as when you have taken a picture in fluorescent light? The cool-warm control is good for regular incandescent light, but not for fluorescent.
Color correction controls are rarely good for correcting white balance, because the color correction will not just neutralize the gray card, but will also color the photo in an undesirable way: usually the blacks get toned or the whites or both. In short one needs some neutrals in an image to set white balance. A white wall or a sheet of white paper will do well; at best add a gray card for the mid tones.