Warming/Cooling by Setting Neutral Gray

Before we get started with this month's feature, the winner of last month's Photoshop Corner contest is:

SONOPP's own John Harper!  He correctly pointed out that after adding a layer to an image and blurring the layer, the best way to allow some parts of the sharp image underneath to show through was to paint with black on the blur layer's mask, and NOT by using the Eraser Tool on the blur layer itself.

Painting with black on the layer mask would allow the underneath layer to show through; but if you make a mistake, you simply paint over the mistake (on the layer mask) with white to restore the blur.  That's so not easily done if you have erased the blur itself.

Congratulations, John!

And now, on to our feature of the month.  This technique is originally due to Dave Cross, Senior Developer of Education for NAPP, and is one of Scott Kelby's favorite tricks.  The basic idea is that if a photograph has a color cast, it will probably affect the midtones much more strongly than it does the shadows or highlights.  Therefore from either the Levels or Curves panel, selecting the midtones eyedropper and then clicking on a point that should be neutral gray, ought to pretty much remove the color cast.

I remember first encountering this idea in one of Ed Pierce's lessons on color correcting, and saying to myself, "How will I know where there is a point that should be neutral gray, i.e., RGB(128,128,128)?  I'd just be guessing!"  Take a look at the image below,

and tell me where you think the neutral gray point should be.  Pretty hard, eh?  But there is a way.  Sort of.

Start by going to the Layers panel and creating a New Layer.  Then from the Edit menu select Fill, and when the Fill dialog appears, choose 50% Gray from the Contents pop-up menu.  After you click OK, set the blend mode of Layer 1 to Difference.

         

Okay, now your image looks like what you see below.  What you're seeing, for each pixel, is the difference between its color and neutral gray RGB(128,128,128).  (Technically, it's the absolute value of the difference, but let's not be picky.)

Now if the image really had a neutral gray point, the difference would be 0, or jet black.  Obviously there are no neutral gray points in the image, as you can tell by checking back with the original.  We'll get back to that.

But for now, choose Threshold from the Create New Adjustment Layer pop-up menu at the bottom of the Layers panel.  Your Layers panel will then look as shown below, and you'll see the Threshold dialog:

         

Notice that the Threshold dialog's histogram does not start all the way over on the left.  That means there are no difference points which are exactly 0, as we had already guessed.  If you drag the slider at the bottom all the way to the left the difference image will turn completely white.  Now slowly drag the slider back toward the right.  Some black areas will appear.  These are points where R ≈ G ≈ B and their differences from neutral gray would have been close to zero had there been no color shift.

Here comes the really neat trick: move your cursor over one of these black areas, and, WHILE HOLDING DOWN THE SHIFT KEY, click on the area.  Voila!  A Color Sampler tool point will be added!  Now just delete Layer 1 and Threshold 1, and this is what you'll see:

I've drawn a red circle to show where the Color Sampler tool point is.  (That's not a point we would have guessed to be close to neutral gray.)  Now let's complete the process.  Choose Curves from the Create New Adjustment Layer pop-up menu at the bottom of the Layers panel, select the midtones Eyedropper (the middle one), and click directly on the Color Sampler point.  Here's what you get:

  

Quite a change.  But does it look a little too cool to you?  If you think so, reduce the opacity of the Curves layer to about 80%, or use two applications of our One-Click Warming method (two clicks of Shift-F3 on my computer):

You may even want it warmer, although the skin tones are good.  But why wasn't it perfect the first time?

Well, there are a lot of assumptions underlying this idea.  For one thing, suppose the original image actually had no point that would have been precisely neutral gray in Standard Intergalactic Daylight.  Then we introduced a color cast when we forced one.  Or suppose there had been such a point, but the original color cast was not linear, i.e., it changed red more than green or blue.  Then a different point might have been the blackest point when compared against a neutral gray layer in difference mode, and when we selected it we introduced an incorrect reverse color cast.  Besides, who says there was a color cast anyway?  That's the way the woman looked in that room under those lights.

In any event, achieving this result in Photoshop would have been more challenging if done some other way.  (For the record, although this image was originally captured in JPEG mode, I've also tried using the temperature corrections available in Lightroom and Camera Raw on this image.  They're a little easier to use, but also more arbitrary.  You get a movable grid showing RGB values and are asked to find and click on a neutral point.  Same idea.  The results are somewhat different but not necessarily better.  Bottom line: it may be that color shifts are partly in the eye of the beholder.)