Digital cameras keep getting smarter, but it seems like every day we receive a friend’s photo in which the color is way off. Any schmo can click the auto correction button in iPhoto or Photoshop, but those results aren’t always the most desirable. Photoshop CS3’s Curves dialog offers a way to manually fix color casts, so you can have a reason to brag about the finished image.

This image from 2003 shows the photographer’s inexperience—the focus is soft in the lower part of the frame, and the camera couldn’t find a white balance.

After correcting the white, the image looks a lot more like it ought to.

What you need: Adobe Photoshop CS3 ($649 à la carte, or with a Creative Suite bundle, www.adobe.com) and a digital photo with an objectionable color cast, containing something you know should be white or gray.

 

1. Stay Informed

The values for red, green, and blue are way off on the chopstick wrapper that we know should be white, so this image definitely needs correction. Another benefit to using the Info palette is knowing the actual values of the colors, which is handy if your monitor isn’t calibrated. Click image to embiggen

Correcting color casts is sort of like landing a plane: It’s possible to do it without looking at flight instruments, but why would you want to? First select Window > Info to reveal Photoshop’s Info palette, which displays information about where the cursor is pointed in an image. The palette contains two eyedropper icons, each with right-pointing triangles you can click. Click them and select RGB color for one and Grayscale for the other. Select 8-bit (0-255) under the RGB eyedropper. Now the Info palette will show you the R, G, and B values for red, green, and blue, the primary colors of light, as well as the K value, which is the percentage of black that would be used if the image were converted to grayscale.

It’s also smart to use cursors that make it obvious where they are pointing. Select Photoshop > Preferences > Cursors. Under Painting Cursors, click Full Size Brush Tip and check the box for Show Crosshair in Brush Tip. Under Other Cursors, click Precise. Click OK to keep the changes.

Finally, make sure the Eyedropper tool isn’t so precise that it samples just one pixel at a time. Select the Eyedropper tool from the toolbox on the left of the window, and its options will appear at the top left of the workspace. Set the Sample Size to 3 By 3 Average (or larger) to prevent one rebellious pixel from ruining the party.

 

2. Analyze the Image

The values for red, green, and blue are way off on the chopstick wrapper that we know should be white, so this image definitely needs correction. Another benefit to using the Info palette is knowing the actual values of the colors, which is handy if your monitor isn’t calibrated. Click to embiggen image

Open your JPEG in Photoshop. Move the cursor over parts of the image, and watch the Info palette to get an idea of how the colors work together to make the image. The Info palette will display the red, green, and blue values of the sample area, with 0 being the darkest and 255 being the lightest. The farther apart these numbers are, the less neutral a color will be. You can also look at the K value, with 0% being white and 100% being black, to learn about the lightness or darkness 
of an area.

Find something you know to be a relatively bright neutral color in real life and mouse over it. You’re after something bright white or gray, but not a highlight, which will probably already be pure white. Look for something with a K percentage of greater than 5, but less than 40 or so. Mouse over it, and look at the red, green, and blue values. If the image has an objectionable cast, the values of at least one of the RGB channels will probably be more than 10 in either direction from the other two.

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3. Fix the Color Cast with Curves

By making the a and b coordinates 0, we make the color into a neutral while leaving the lightness intact. This is smarter than making up a neutral value for the color. Click to embiggen image

Select Image > Adjustments > Curves to open the Curves dialog. Check the Preview checkbox, and uncheck the Show Clipping checkbox. If the Curve Display Options dialog is not visible, click the downward-facing triangle to make it appear. Make sure it’s set to show the amount of light, rather than pigment, and that all the checkboxes are checked.

Double-click the rightmost Eyedropper icon in the Curves dialog to reveal the Set Target Highlight Color dialog, which sets what the Eyedropper tool will do to the image the next time you click it. The dialog represents the same color in four ways: Hue, Saturation, Brightness (HSB); Red, Green, Blue (RGB); L, a, b (Lab; see “The Power of Lab Color Correction”); and Cyan, Magenta, Yellow, blacK (CMYK). Don’t type anything yet: Doing so is tantamount to guessing, or eyeballing, color, which we’re trying to avoid.

Click the spot you think is supposed to be neutral, to sample its value into the dialog. Now, don’t move the mouse, because we want to keep the cursor exactly at this spot. Press the Tab key until you get to the a field of Lab color, and change a to 0 (zero). Hit Tab again and change b to 0 (zero). Press Return to accept the new target color.

If you haven’t moved the mouse, click the mouse button to cause the new color to be applied to the spot that is supposed to be neutral and to adjust the rest of the picture. If you like the change, press Return to accept it, then tell Photoshop not to save the new target value as a default in the prompt that follows. If you don’t like it, pick a different neutral spot and try the Set Target Highlight Color dialog again.

Once the color is corrected, you’ve got a good starting point for any further work on the image.

 

The Power of Lab Color Correction

The L, a, and b values in the Select Target Highlight Color dialog represent Lab color.

There are many ways of arriving at a neutral value. You could just as easily have copied one of the numbers in R, G, and B into the other two values to get a new neutral, but how do you know if you chose the right number?

Lab color solves this problem by separating the color into three different values: L, or lightness, and a and b, which are their positions between red and green, and yellow and blue, respectively.

Think of the detail in an image as differences in lightness. Whether the photo is black and white or color, it’s hard to tell what the picture is about unless it has varying degrees of lightness. By not changing the Lightness of the neutral sample, the color correction is more faithful than it would be if the sample’s lightness were also changed.

 

Red, Green, and Blue?
Many people learned the three primary colors, which can be used to make almost all other colors, in elementary school art class: red, yellow, and blue. But there are actually two sets of primary colors—subtractive and additive.

The red, yellow, and blue subtractive primaries that the art teacher had you memorize in kindergarten are for pigments, which absorb (that is, subtract) some colors of the light that hits them and reflect the rest back as the colors we see.

When working with light, as with your digital camera, we use the additive primaries: red, green, and blue. They’re called additive because when we project (that is, add) red, green, and blue lights together, they blend to make white light. The art teacher probably didn’t tell you about additive color because you were playing with finger paints, not flashlights