The Zen of White Balance
The real purpose of white balance adjustment is to compensate for the color temperature of the prevailing light source so that your image will show accurate color. Whether you do so by making adjustments on your camera before you shoot or on your computer after you shoot, the objective is supposed to be the same. But rather than stopping there, let's look at some other, more creative uses for white balance adjustment.
We all know that late afternoon sunset light makes things look orange while open shade at mid-day gives things a blue cast. But as we saw the week before last, sunset has a color temperature of around 3200 Kelvin while open shade is up around 9000 to 20,000 Kelvin. In other words, contrary to what seems common sense, higher temperatures are blue while lower ones are orange. We usually associate blue with cold and orange with hot, but not in this context. As things heat up, they first glow orange and only start to glow blue when they get really, really hot.
To doubly confuse things, even in photography we still use the term "warmer" to mean "more orange" and "cooler" to refer to an image that is "more blue."
What this means is that if we want an image to appear warmer, we need to shoot it when the color temperature of the light is lower, and vice versa. Given that coming back at a different time of day isn't always an option, film photographers have long used warming filters for this purpose. Their use is fairly straightforward. They have an orange tint to them and when you hold one over the front of your lens, they give the images it records that same orange tint.
While the same can be done with a digital camera, there's really no need since we can adjust white balance electronically. Moose Peterson and others pioneered the mantra of shooting at "Cloudy -3" white balance to impart a warm tone to their images, even when shooting at mid-day. But overcast (cloudy) skies have a color temperature of 6500 Kelvin and up, which we know is actually bluer than color temperatures that are lower than it. Indeed, the higher (more blue) we set the color temperature on a camera, the warmer (more orange) the results will become. This is because by setting the white balance to a specific color temperature, we are telling the camera the light is that color. To compensate, it artificially biases the result in the opposite direction. Thus, "Cloudy" conditions may mean cooler lighting, but setting your camera this way when the circumstances aren't really overcast can result in images with a warmer tone to them.
This is definitely one of those things that seem to make sense on the surface and only start to get confusing once you look at them more closely. A cloudy day provides light with a higher color temperature that results in images with a blue cast that we call "cooler," yet setting a camera to Cloudy white balance when it isn't really cloudy can make images with an orange color cast to them that we call "warmer" when in fact what we have really done is to artificially lower the color temperature. Read that last sentence a few times until it starts to make sense, then read it an extra time for good measure. If you think you've got it, try contemplating why we use such confusing and contradictory sounding terminology to describe something that really ought to be more simple.
At any rate, the bottom line is that the camera will compensate for a change in how we set the white balance by adjusting the look of the resulting images in the opposite direction, regardless of the names we give to either the color temperature or the resulting look.
For fun then, imagine that you are using manual white balance but instead of using a gray card or even a white card, you use a red card. The camera will see it, and compensate for what it believes to be the color of the light source by imparting a cyan cast to any images shot with tit. Likewise, white balancing with a green card will yield magenta images, and a blue card will give yellow results. Whatever color is opposite that color on the color wheel is the color you will end up with. This can of course be used to create pop art images, but the idea does have practical applications too. A site called warmcards.com sells cyan/blue tinted cards designed for white balance applications where the photographer wants to give their images a warmer tint. They also sell "green cards" intended to compensate for fluorescent lighting. Film filters for this purpose are traditionally magenta in color to counteract the green of fluorescent lights, but given how things work with digital white balance, a green card will cause the camera to compensate towards the opposite (magenta being the opposite of green), yielding the same effect.
All this holds true once you get your images onto a computer as well. Moving the color temperature slider to the right means increasing the color temperature, which we now know implies that the light source had a bluer cast, yet it results in warmer (more orange tinted) images. The computer, just as the camera, compensates by adjusting the appearance of an image towards the opposite of how we set the color temperature slider. In order to creatively alter an image by giving it a warmer hue than the light under which it was shot would have naturally done, you only need to increase the value the color temperature slider is set to (towards the blue end, although Adobe kindly refrains from confusing people by actually coloring the user interface).
For truly creative effects, Adobe Camera Raw lets you use an eyedropper to select an object in an image that should be neutral colored. In practice, this works sometimes, but not often for nature photography given that the great outdoors comes in such a variety of hues that rarely are truly neutral and we generally want to capture the golden color of the setting sun as it bathes the landscape, not neutralize it. If you use it to click anywhere on your image though, ACR will assume that that area is supposed to be neutral and will correct all the other colors accordingly. Thus, if you click on a green object such as a tree, you will impart a magenta cast to your image, and so on.
The composer John Cage tells a story of the Zen teacher D.T. Suzuki who was quoted as saying that "before studying Zen, men are men and mountains are mountains. While studying Zen, things become confused. After studying Zen, men are men and mountains are mountains." After telling this, Dr. Suzuki was asked "What is the difference between before and after?" His reply apparently was "No difference - only the feet are a little bit off the ground." I think the same thing could be said of white balance. It all makes sense at first, but once you start investigating how it really works, things seem a bit confused. When you actually put it into practice though, it all makes sense once again. Hopefully, you are closer now to being on the "after" side of this puzzle and can make use of white balance creatively rather than simply as a way to achieve accurate color. Sometimes, accuracy isn't what we are really striving for.