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After first using Levels - Histogram and then Curves to adjust exposure (see Part II and Part III), the next tool you might want to use is Color Balance. The Color Balance tool lets you modify color hues in an image. For example, if a shot taken inside your house under fluorescent lights is too green, or one under incandescent lights is too red, or the dark blue in deep water off the edge of a wall has a little too much purple or magenta, then say hello to Color Balance.
To use Color Balance you should understand a little about how colors are generated on your computer. Computer monitors make colors by lighting Red, Green and Blue elements, and you get other colors by mixing the brightness or intensity levels of the additive primary colors red, green and blue. As black and white are "opposite" or inverse colors, so red is opposite to cyan, green is opposite to magenta, and blue is opposite to yellow. I.e., if you have an image with the colors black, red, green and blue, and you invert the colors using Photoshop or another image editor, then black turns into white, red into cyan, green into magenta, and blue into yellow:
Figure 1
In Photoshop, click on "Image" on the menu bar, then "Adjustments" (or "Adjust") and "Color Balance..." and here’s what the Color Balance dialog box looks like when you first open it (Figure 2):
Figure 2
So if for example you wanted to take a bit of green hue out of a picture, you would move the middle slider to the left, away from Green towards Magenta. That would turn down the Green and add in some Magenta. Also note, you click in the Tone Balance box to select Shadows to modify the darkest colors, Midtones to modify the midrange colors, and Highlights to modify the brightest colors. So you have a total of 9 sliders you can play with: 3 sliders in Shadows, in Midtones and 3 in Highlights.
If you wanted to add or reduce a color that isn’t one of these primary colors, Red, Green, Blue, Cyan, Magenta or Yellow, then you’d have to move more than one slider. For example, orange is a combination of red with a little green, and possibly a touch of blue as well depending on your shade of orange.
Here’s a picture of Bloody Bay Wall at Little Cayman I’ll use for a Color Balance demonstration (Figure 3):
Figure 3
If I decide that the deep water and the wall in the bottom right of the picture looks a little too purplish, I could go right to Color Balance and start moving the sliders with the Preview check box on, to see how I could change it. Optionally, first I might use the Eyedropper tool to sample the color that I’d like to change. Then I could look at the color palette information to help me decide how to change it.
Say I use the Eyedropper tool 
to click on a dark region near the bottom right that looks a little too purple for my tastes (1 - Eyedropper tool circled in red on Figure 4) and so I get that color in the color palette. I click on that color in the palette (2 - red arrow) and I see the Color Picker dialog box that shows me some numbers about that color (3 - partial information that is displayed in the Color Picker dialog box):
Figure 4
You don’t need to be know exactly what those numbers mean, just notice which are larger and which are smaller in relation to each other. In the RGB column (Red/Green/Blue), I see Blue has the highest number (81). This tells me there is more blue than red or green, which makes sense because I know it should be blue there in the water. In the CMYK column (Cyan/Magenta/Yellow/Black), the highest number is Cyan (100%) which ought to be okay (cyan color is a mix of blue and green), but the next highest number is Magenta (97%). So these numbers give me a hint that the purple I’d like to get rid of might be due to a little too much magenta.
If you didn’t follow that brief introduction to the Eyedropper tool and Color Picker, don’t worry, its not necessary. You can skip it and go right to Color Balance and start experimenting with the sliders. But Photoshop and some other image editing applications do give you that kind of information if you care to learn how to use it.
Now I open the Color Balance tool and try moving the sliders around to see what happens. At any time to reset all the sliders, you can hold down the Alt key on a PC or Option on a Mac which changes the Cancel button to Reset, and then you can click Reset.
The area in the Bloody Bay Wall picture I am concerned with is pretty dark, so as shown in Figure 5 I click on Shadows in the Tone Balance box at the bottom. I move the sliders a bit and finally settle on moving the Cyan/Red slider to -3 towards Cyan, away from Red, and moving the Magenta/Green slider to +11 towards Green, away from Magenta. I also selected Midtones and decided to shift the Magenta/Green slider to +4 towards Green, away from Magenta:
Figure 5
That got rid of some of the purple in the deep, and it also made the coral on the near wall slightly less red and a little more green. If I wanted to modify only the deep water in the bottom right of the picture, I could use one of the selection tools such as the Lasso to select only the areas of the image I want to change. Then I would Feather the selection ("Select" on the menu bar, then "Feather...") by some number of pixels so my changes will blend in smoothly with unedited pixels nearby, and then I would use Color Balance only on the pixels I selected.
Experiment with Color Balance. Start by moving the sliders one at a time all the way to the extreme ends and see how they work, see how it can make very bizarre changes in hue. Then back off the sliders and try smaller changes to find more subtle and natural differences in color. Next time we’ll look at the Hue/Saturation tool which can give you finer control over specific colors.
Previous: Digital Imaging Editing - Part III
Next: Digital Imaging Editing - Part V
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