White balance correction means to adjust the illumination in a image to white. In order to do this one must have some neutrals in the image like a gray card or a white sheet of paper. Software can adjust white balance, but not all software solutions are equally good.
White balance is the first thing you want to work on when you begin to retouch your photo. White balance refers to the color of the light and implies that the best light color is white. Some photos, 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 a good solution, since auto levels just sets the darkest pixels to black and the brightest to white without considering the mid tones. But what if the brightest pixel in your photo is not white? Or what if you do not have pure black in the photo? (Most pictures 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.
When working with RAW images, you will have found that the RAW converters generally come with a control for color temperature, which means a slider 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 neon light? The cool-warm slider is good for regular incandescent light, but not for fluorescent.
Color adjustment sliders are rarely good for correcting white balance, because the color correction will not only neutralize the gray card, but will also color the picture in an undesirable way: often the blacks become 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; preferably add a gray card for the mid tones.
White balance is the first thing you want to work on when you begin to retouch your photo. White balance refers to the color of the light and implies that the best light color is white. Some photos, 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 a good solution, since auto levels just sets the darkest pixels to black and the brightest to white without considering the mid tones. But what if the brightest pixel in your photo is not white? Or what if you do not have pure black in the photo? (Most pictures 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.
When working with RAW images, you will have found that the RAW converters generally come with a control for color temperature, which means a slider 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 neon light? The cool-warm slider is good for regular incandescent light, but not for fluorescent.
Color adjustment sliders are rarely good for correcting white balance, because the color correction will not only neutralize the gray card, but will also color the picture in an undesirable way: often the blacks become 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; preferably add a gray card for the mid tones.
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For reference, I based this article on these two: this article about White balance and this one White balance