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Use Curves to remove color casts from your images

Added on Thursday 8th of September 2011 02:44 am EST

Use Curves to remove color casts from your images
by Renée Dustman

Adobe Photoshop CS2/CS3/CS4/CS5
Operating Systems:
Macintosh, Microsoft Windows

There are many things to consider if you’re to figure out why your image has a color cast, especially if it didn’t start out with one. But the bottom line is, when that happens, your focus should be on fixing the problem.

To remove color casts from images, we’ll:
Offer a brief explanation of what a color cast is and the typical causes that bring it about.
Introduce you to that panel and the different options that are shown there.
Show you how to use the Curves feature in Photoshop in conjunction with the Info panel to cast away that nasty color cast from your image.

There you sit, staring at the digital image that you scanned in (or downloaded from a digital camera), which in no way looks like the original photograph (or what you saw through the camera lens). You continue to stare at your monitor hoping that, if you do so long enough, the color cast that has mysteriously appeared in your image will disappear in the same way, but it doesn’t happen. Photoshop is good, but it isn’t that good. You’re going to have to help a bit if you want your image to look picture-perfect.


Preliminary considerations
Color casts aren’t really such a mystery; there are many reasons for their occurrence. In fact, a color cast often stems from the original photograph and simply becomes intensified to the point of recognition once digitized. If you look closely at your original photograph, you’ll probably see that the problem existed before the scanning process. Other accomplices that cause discoloration include the input device (scanner or digital camera), color settings and the output device.
For now, just know that in order for the color in your image to match that of the original, accurate and consistent device settings are required. However, there are far too many input/output devices on the market today for us to give you specific settings for your particular situation. If your image is to be printed by a service bureau, the prepress operator can provide you with the correct color profile and resolution settings.
What we can tell you is that it’s almost always advisable to work with digital images in RGB mode and at the highest resolution you might ever need them to be. This is so your images always start out with the most digital information they can possibly have. Then, if necessary, that information can be adjusted to match the capability of the output device. Just remember: You can always decrease image data, but you can’t add to it what was never there.


Is that hue?
The first thing you should always do when you notice your image doesn’t look right is to rescan it. If after you’ve rescanned your image the color cast remains, then it’s time for a little Photoshop intervention. However, before you can do this, you need to determine that what you’re seeing is actually a color cast and not something else.
As shown in Figure A, a color cast is typically pinkish gray in color—though a yellow or green cast sometimes occurs as well. It also typically affects the entire image, not just one area. Gray balance, or lack thereof, is more of a factor with a color cast than of misplaced hue.



Finding a neutral gray
The first step in readjusting the gray balance and thereby removing the color cast in your image is finding a neutral area—that is, something that should have an even balance of red, green and blue. Once you determine this, the next step is to record the red, green and blue values of that area, which will display in the Info panel as you move the Eyedropper tool over the image.
To begin, select the Eyedropper tool and choose 3 By 3 Average from the Sample Size pop-up menu in the tool options bar, to make sampling the area easier. Now, position the Eyedropper tool over the area that you’ve determined should be neutral gray. Then look at the Info panel (Window > Show Info) a...