This same idea was adapted to digital cameras and is a tremendous help. With film, matching colors to a scene often required extensive filtration, and even then, you couldn't be sure. With digital, you can count on colors being accurate, especially since you can see them right away as you shoot. While the LCD monitor won't give completely accurate colors, it will give you enough of an idea of what's in the image file that you can judge if your white balance is doing the job.
It's interesting to note that at all major sporting events covered by the networks, they use their big video trucks with hundreds of thousands of dollars of technology, while the network camera people still use manual white balance; it's called "painting" or "shading" in the business. Do they know something we don't? Why not just use auto white balance? After all, auto white balance uses complex algorithms and internal camera processors to examine a scene and try to match the manual adjustment of white balance. Manufacturers have become very good at this, making auto white balance a valid tool. Still, many pros prefer selecting specific white balance settings for several reasons:
1. Auto white balance can shift as a lens is zoomed or the composition changed across a scene. This can make matching images more difficult when working with them later; this wasn't a problem with film, as film had one and only one "white balance" matched to a specific color of light.
2. Having a specific white balance makes colors more predictable. This is just like shooting a specific film; you know what the colors will look like.
3. The range of color temperatures balanced by a camera is generally greater manually than on automatic.
4. Scenes usually are completely cleaned up from overall color cast problems (some color casts, such as sunset colors, are supposed to be included).
5. Panoramas need consistent colors; auto white balance can change enough that time must be spent balancing different frames in the computer.
6. RAW workflow is more efficient because you chose a white balance specifically for the scene; each image from that scene will have a consistent color.
Although nearly all digital cameras, from small to large, have a custom white-balance control that works like the videographer's white balancing on a white card, few photographers, amateur or pro, know much about it, and fewer still actually use it. It's worth learning even if you don't use it all the time. It offers some control and color correction not possible any other way. There are some reasons beyond old habits that even video folks use it. Let me give you some examples.
While leading a photo tour to Peru last June, I learned one of the participants was having trouble getting clean colors from a unique area of salt evaporating pools. Everything kept looking blue (this may have been from the altitude, although her camera reacted more strongly than others). I showed her how to use the camera's custom white balance and instantly the white salt looked white! The whole scene looked better in her photos.
At Machu Picchu, we found ourselves in a rainstorm unusual for that time of year. As it let up, everyone began photographing again, but the gray conditions gave less than satisfactory color. Several people tried custom white balancing and discovered they could get much better color.
One area where auto white balance has given me problems is in the shade. Shade colors are extremely varied, yet whenever I've done comparisons from auto white balance to shade and cloudy presets to custom settings, I've almost always preferred the custom settings. Custom settings let your camera precisely measure the color of the light and adjust the camera accordingly.
Auto white balance is somewhat of a compromise (although often a good one) based on what engineers think a picture should look like. It just doesn't always give the best colors possible. To me, custom white balance furthers the idea of "getting the shot right" from the start and supports the craft of photography. It also allows you to interact with the subject, while you're still with the subject, to be sure you get the right colors.
Hard-core RAW users might say they always can match anything done by the camera later in software. This isn't exactly right with custom settings (unless you photographed a white object and kept that photo to use as a reference). Even if you're using RAW, the use of custom white balance often will result in better colors because you're actually measuring the light's effect on a white card rather than trying to figure out what white balance should have been after the fact.
Finally, you can tweak colors in unique ways that can make your work more personal and controllable. You can white balance on a color, for example. If you white balance on a pale blue card, say, the camera removes that blue and warms up the photo. You can white balance on any color you want and the camera will try to remove that color (it will vary in how well it does that because the control wasn't really designed for this!). It's funny to see the card become more "neutral" if you take a picture of it (or see it in the live LCD of a non-SLR digital camera). You can experiment by picking up a bunch of paint sample cards from your local paint store.
Using custom white balance isn't difficult. Put a white (or gray) card in front of the camera, press the appropriate button and the camera is matched to the scene. I wish I could give more specifics, but unfortunately, this is one of those areas where engineers seem to want to express their creativity. Custom white-balance procedures should be consistent, but they aren't. Cameras vary considerably in how this setting is achieved, even among the same brand.
First, you need to set up the custom white balance, then select it among your white-balance settings for actual photography. Setup is usually menu-based. You'll need to check your manual.
Place something white in front of your camera (it doesn't have to be in focus unless your camera won't work otherwise). Be sure it's in the same light as your subject. For digital cameras that have live LCDs (non-SLR), you usually go to the white-balance setup control and move your camera until a marker lines up with the white and press a button (which button depends on the whim of the camera designer, it seems; sometimes it's marked on the LCD, sometimes you have to refer to the manual).
For digital SLRs, you have to take a picture of the white object. The next steps are quite variable, but typically you play back that photo and push some button or combination of buttons to lock in a white balance based on it. (Doesn't this seem silly? There's really no good reason why this couldn't be standardized.)
Once you've set the white balance, you need to be sure you've chosen it among your white-balance selections.
Give it a try. Pick up some paint sample cards and try them, too. Custom white balance is a great way to increase your control over your image while you're still in the field shooting. Plus, it often will reduce the time you spend in front of the computer later.