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Removing a Color Cast in Photoshop  - or

Photoshop Color Correction Using Curves

category: digital photography and image editing

Some images have a color cast that you may want to remove. Auto Color does a good job with many images, but not all, and it can be done nondestructively with a Curves, or Levels, adjustment layer. Alternatively, you can remove a color cast manually with Curves, but it's more work and any small error will have you starting over, while Color Balance is difficult guess work because the eye is used to ignoring most color casts. The best approach, bar none, is to use the Eyedroppers in the Curves dialogue box augmented by Threshold and the Info Palette. This is how it's done with a minimum of guess work.

Open a digital photograph or a scan in Photoshop. Run Levels, Auto Levels is probably good enough, so that the contrast is reasonable, and let's start by introducing an undesirable color cast that we can remove. Click on the Ajustment Layer icon at the bottom of the Layers palette and choose Photo Filter. Choose magenta from the Photo Filter Adjustment Layer drop-down and click OK. You might get this kind of color cast outdoors on a cloudy day. This color cast is not attractive, so let's get rid of it.

Create a new Threshold Adjustment Layer above the Photo Filter layer, and move the slider to the far left. Now, move the slider back toward the center slowly, use the arrow keys if you like, and when the first blob, not speck, of black appears on the white background, move your cursor to that area, hold down the shift key and click. This will mark the darkest area of your image with a Color Sample, and the Info Palette will record the RGB values of that area. Next, record the lightest area of your image by moving the slider all the way to the right, and then slowly back toward the center until white appears on the black background. Again, holding the shift key, move your cursor to the white area and click.

Having recorded the location of the lightest and darkest areas of your image, you're finished with the Threshold Adjustment Layer. You can click OK, and save the layer for future reference; you will have to make it non visible, or just click cancel and it will disappear.

Now to adjust the midtones we will need a gray area; click and hold the Eyedropper icon in the toolbox and, from the Fly Out menu, choose the Color Sampler icon. Zoom in on your image to 100 percent or more, and find an almost gray area that is five or ten pixels in size. The closer this area is to 50 percent gray or middle gray, reading about 128 in the RGB values of the Info Palette, the more accurate your adjustment will be. When you find a good candidate for gray, click on it and you'll see its RGB values recorded in the Info Palette. The numbers representing the RGB values will not be the equal, as gray should be, because of the magenta color cast. My gray sample was 116,124,134 in the Info Palette RGB values. Making these values equal, or neutral gray, is the key to eliminating the magenta color cast.

Let's make the adjustment. Click on the Adjustment Layer Icon at the bottom of the Layers Palette and choose Curves. Remember, this Adjustment Layer has to be above the Photo Filter layer in order to effect it. First press your Caps Lock key to turn your cursor into a crosshair, then, from the three eyedroppers in the lower right area of the Curves dialog box, choose the black one on the left. Place your crosshair cursor above your number one color sample, marking the darkest area of your image, and click.

What just happened was that Curves, via the black eyedropper, just converted the darkest area of your image to black, eliminating any color cast in the shadows of your image. Black is RGB 0,0,0 and is therefore neutral. Get it? There is no more magenta hiding in the shadows. Now do the same thing with the white eyedropper, this time clicking on your number 2 sample and neutralizing the highlights of your image to white, or RGB 255, 255, 255.

Now we'll use your gray color sample to adjust the midtones. Click on the middle eyedropper and then on your number three color sample, and your color cast should be completely gone. Finding the best gray in your image is a judgment call, so if you'd like to give it two or three shots just press Ctrl+Z to undo and try a different spot, all the time keeping a close eye on your image.

If you're not sure what you're looking at any more, just uncheck the Preview checkbox and check it again to see a before and after comparison. If you decide you want to start over, press the Alt key, and the Cancel button, in the dialog, will change to Reset; click it.

He said on 10.08.05 @ 05:58 PM CST