Color Balance Adjustment with Curves
So now that we've covered how to adjust color balance in Photoshop both with the appropriately named Color Balance tool and with Levels, it's time to spend one final week on the subject. In case you hadn't already guessed, you can also correct for color casts with Curves, and it is this that is the subject of this article.
First, brief review of how Curves in general works. The dialog consists of a grid or graph with the axis across the bottom representing input values from black to white and the vertical axis representing output values with the same range of possible values. In 8-bit per pixel mode, zero represents black and 255 represents white. Initially plotted on this graph is a diagonal line from the bottom left corner to the top right. This line is your curve, or at least it will become your curve. Curves transforms each pixel by locating it in terms of brightness or luminosity along the bottom axis, then going up until you hit the curve line, then going across to the vertical axis to find out its new value. Since the initial curve is in fact nothing more than a straight line diagonally across the grid from one corner to the other, this pixel by pixel transformation changes nothing. Every input value is mapped via this line to itself: zero maps to zero, 255 maps to 255, and every value in between likewise maps to itself.
The magic of Curves though is that you can move, bend and ... well, "curve" this initial diagonal line. Click on it with your mouse and you can push it and pull it as desired. Pull it upwards and the value at that point on the bottom axis now maps to a greater value on the left hand vertical axis. Push it down and the output value will be less than the input. In this way you can make selected input values transform to brighter or darker output values.
All this still holds true when working on an individual color channel with the exception that black and white now become simply none and lots of the selected color without regard to how much of the other two color channels is present. Assume you select the Green channel for instance. The horizontal axis for input values now ranges from no green (zero in 8-bit mode) on the left to lots of green (255 in 8-bit mode) on the right. So too does the vertical axis from bottom to top for the output values. To make your image greener, grab the curve line with your mouse and pull it slightly upward. It won't take much to effect a visible change. Pull it too far and you'll get way too much green. If instead you push it slightly downward, your image will turn less green.
If you have a green cast in the shadows, you can grab the curve on the bottom part and adjust it. If the change you want to make is in the highlights, you can move the upper part of the curve line. Or both — whatever you need for adjusting the color balance. The curve for each channel can be adjusted individually.
In some ways, Adobe made Curves more user friendly than Levels. When selecting a color channel, the new histogram display added in Photoshop CS3 displays in the color of the selected channel whereas Levels always displays in black and white even when you are working on only one color channel. Also, Levels makes you move the midpoint slider right to decrease the amount of the selected color even though most of us generally think of values increasing from left to right rather than the other way around. In Curves, it's a lot easier to remember since you intuitively pull the curve line down to decrease and up to increase color.
As always, these changes are best made on an Adjustment Layer rather than directly on the image data so that you can change your mind freely later and fine tune things without degrading things. And as with Levels, you should change the Blending Mode to "Hue" when using a Curves Adjustment Layer for editing color balance so as to not affect the overall luminosity of your image.
Most images probably won't require Curves for adjusting color balance unless you are shooting under controlled studio conditions and need to ensure exact color. But just because you may not need Curves doesn't mean you can't use Curves if you want to. They truly do provide the ultimate in control for fine tuning things.