Down to Brass Tacks Let's cut the crap.

Cancelling color casts

Color casts (overall color drifts) are avoided by using the appropriate white balance setting at capture time (the most accurate of which is a "custom" white balance), or by including a neutral reference in the picture for adjustment in post-production (such as a WhiBal).

Unfortunately, we don't always have time to follow this procedure, so we must often correct color casts arbitrarily in post-production. The problem is that it is not always possible to find parts of the subject which are of a neutral color, with which we can cancel the color cast. And even when we think something is neutral, chances are it is not really (who says this white wall was really pure white — it probably was yellowish!)

The quick and dirty solution is to pick something "close enough" as the reference for setting the white balance, and to adjust, subjectively, until we get something that looks more natural (completely neutral images often look unrealistic, too cold or too warm).

There is a trick to fix color casts in Photoshop which involves:

1. Duplicating the image's layer;
2. Applying an "Average Blur" filter on that layer (this reveals the overall color dominance);
3. Inverting that layer (to obtain the opposite color of the cast, the one that neutralizes it);
4. Changing the "Blending Mode" of that layer to cancel the color cast of the image.

Obviously, this trick can only get you so far — you still have to judge by eye what "feels right", by adjusting that layer's opacity, to apply more or less of the cancelling.

That being said, the procedure can be optimized to be executed faster and to render smaller files. Here's the trick. With a Smart Object of the image:

Apply an "Average" blur Smart Filter:

Create a "Curve" Adjustment Layer and use the "Gray Point" eyedropper anywhere on the image:

In a single click, you have set the three color curves to perfectly neutralize the average color of the image!

To finish up, simply delete the "Average" blur Smart Filter, which you don't need anymore:

You end up not with a new full-size layer of a fixed color (taking up disk space uselessly), but with a much more flexible "Curve" adjustment layer.