In my last post, Amy asked how to fix a color cast when you don't have all three colors to sample (i.e. black, gray, and white). There are many possible ways to do it, but I'm going to stick with the method from my last post, except that this time I will use only one sample: the white point. So here is a modified version that use the same photo from the last post:
This is the "before. It has a blue color cast and it's too dark.
Step 1. This step only applies when the photo is too dark (and I could have done it in my previous post). In order to get a better look at the problem, I lighten the photo by duplicating the background layer and setting the blend mode to "screen" as described in my "fix dark photos" tutorial.
Post "screen" layer, the photo is much better, but it still needs to move away from the blue hues.
Step 2. Add a curves layer and use the "white" eyedropper on the whitest area of the photo. Since the screen layer has already lightened things up, the white sample will not produce a result as extreme as it did in my non-screen method.
Note how only one adjustment warmed things up substantially. If your results are too strong, you can always dial down the opacity of your curves layer.
step 3. Add another curves layer and adjust the individual "blue" and "green" channels (in that order—and "red" if you must, but I would avoid it) as described in my color and tone tutorial. On the same curves layer, you can use to pull-down menu to go back to the full "rgb" curve and add a slight "s" curve to increase contrast. Once again, dial to opacity down to get results you like:
Note that my second curves layer is set to 73% opacity, this is because I thought the color and contrast was pretty good, but a little too strong. Rather than redo it, I simply toned it down by lowering the opacity. Once I have made my curves adjustment, I am satisfied with the final result:
This last step has improved the skin tones. The results are very subtle and will vary according to your monitor. If you want to, you can add curves layers indefinitely and adjust opacity as needed. It may not be the most elegant solution, but it works.
For me, the main point here is twofold:
1.The individual rgb channels are a powerful tool that can still get you great color even in the worst of circumstances.
2. The sometimes less-than-ideal results you get from using only one eyedropper can be fixed by adjusting the opacity and using more than one curves adjustment.
If you want to add one more principle, it would be to use masks+curves adjustments+opacity changes. You do some pretty amazing retouching with nothing but those steps.