I wish I could say I invented this technique for correcting color cast, but I actually learned about it from an online tutorial. Here I’ve added a wrinkle that is helpful for people like me who haven’t upgraded their Photoshop in a while.
We’ll start with this image of the multimedia center at the main branch of the San Francisco public library.
The first thing we need to do is to duplicate the image.
What we’re going to do with the duplicate image is average all of its color. We will use the resulting value to determine our midtone gray point. If you have a recent version of Photoshop you can use the “average blur” filter to do this. I have an older version, which doesn’t include that filter. What to do?
The answer is to resize the duplicate image to a single pixel. This will give the average value. (You will need to enlarge the view of the one-pixel image a little so you can use an eye dropper on it, as we’ll see.)
Now we’ll open a threshold adjustment layer.
With this we find the black point and white point. We do this by moving the slider all the way to the left and then backing off a little until black areas appear. One of these will be our black point. We mark it by shift-clicking (this marks the spot with the color sampler). Then we move the slider to the right, back off, and mark our white spot.
Once we’ve marked our black and white points the threshold adjustment layer is no longer needed, so we can cancel out of it and open a levels adjustment layer. The levels panel has three eyedroppers that when clicked on the image will set the black, gray, and white points. With the eyedroppers on the left and the right we set the black and white points, using the spots we identified previously with our threshold adjustment. Now we want to set the gray point, and this is where the duplicate image that we used to find the photo’s average color comes in.
For most images this technique really works well. Of course, normally you will still do your usual adjustments and sharpening if you’re really going for the best result. (I haven’t done that here.) The original image is on the left and the image with the color cast removed on the right.