One of the best ways is to take a photo of the X-rite ColorChecker chart. Then use a polynomial regression to fit each color channel to the desired colors, sch as the sRGB values of the color checker chart.
An alternative ad-hoc way is to locate the bright region of your image, and assume that should be white (red=green=blue), then linearly scale each color channel so that the gray level of the brightest pixels end up all the same (say they all = the mean of the 3 color channels in the bright portion). This is by far the easiest (but worst) way to color correct.
Of course there are more sophisticated ways, but I don't know how fancy you want to get.
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