Every once in a while, while referring to the creation of multiple photographs with the intention of compositing them together in a finished blended image, someone will use the phrase "shoot it on a green screen." The term "green screen" tends to be loosely interpreted, but most folks are talking about the approach of working with green or blue background colors for easy removal ("keying") in video production. The most common approach is seen in your local television weather report, where the friendly weatherman stands in front of a giant map of the United States. He's actually standing in front of a large green screen, and with the click of a single key the video processing software masks out anything bright green and replaces it with another image—i.e., the weather map. It's simple and effective, and perfect for video—wonderful, tremendous and fantabulous, even. But, it's not ideal for photography. In fact, when photographing people or things for the purposes of compositing, you shouldn't photograph them in front of a green screen. Here's why.
You see, the trick with green screen for video is that the scene contains moving elements—if nothing else, that weatherman standing there isn't standing perfectly still. So, the producers need a fast fix for keying the subject from the background. They chose bright green as a color that's not seen in humans, and not common in our clothing, either. With a single mouse-click the bright green is keyed out and replaced in processing with a new background image.
There are challenges with working with all of this green. The biggest problem, of course, has to do with lighting that bright color. If you position your subject too close to the background, green light reflecting from the background can give your subject (or even just his or her edges) a subtle, sickly green cast. Worse, even if you do allow enough distance between subject and background, shiny surfaces like shoes, buttons and eyeglasses can still reflect that green background—and that ain't good.
In the still world, we don't need a single-click solution to masking subjects out of backgrounds. Sure, convenience is nice, but with great tools like Photoshop's Magic Wand and Quick Select tool, it's not exactly a long, time-consuming process to separate subject from background. The green isn't necessary, and for the reasons outlined above, it's risky too.
So, what color should you use in the background of an image destined to become a composite? Simple: match the color of the background to the color you're going to drop in. Neutral tones like white, gray and black will ensure that you don't reflect a color cast onto your subject, and precise color choices (say, blue if you're going to composite your subject against a blue sky, or red if the subject is destined to be dropped in front of a red brick wall) mean that any color cast you do create isn't a detriment, and can actually add to the overall look.
More than anything, though, matching the color of the background you use to the actual color of the eventual background means you'll have less trouble blending the two images seamlessly. No more glowing green halo edges, or an outline of stray white pixels. Now that subtle little color remnant, even if it's just a pixel wide, will help blend the subject with the background rather than spoil the illusion and give away that the work is a composite.