We primarily use green for green screens because it's the color furthest away from most skin tones, minimizing the chance of the background removal process affecting the subject.
Here's a more detailed explanation:
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Color Distance from Skin Tones: The primary reason for choosing green (or sometimes blue) is that these colors are relatively rare in human skin, hair, and clothing. This large color difference makes it easier for software to isolate and remove the green background without accidentally removing parts of the subject. If a color close to skin tone were used, portions of the subject might be removed along with the background during the "keying" process.
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Color Keying (Chroma Keying): The process is called "chroma keying," where a specific color range is selected and made transparent. Any area of the video or image that matches that specific green (or blue) is replaced with another image or video.
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Alternatives to Green: While green is most common, blue screens are also frequently used. The choice between green and blue often depends on the specific shooting conditions and the subject's clothing. For example, if the subject is wearing green, a blue screen would be more appropriate.
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Technical Advantages (Historically): Early digital video cameras were more sensitive to green light. This meant that green screens often provided a cleaner, less noisy key. Although this is less relevant with modern cameras, the practice has remained standard.
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Practical Considerations:
- Availability: Green screens (and blue screens) are readily available and relatively inexpensive.
- Lighting: Green and blue tend to be easier to evenly light than some other colors, although proper lighting technique is crucial for a good key, regardless of color.
In summary, the use of green for green screens boils down to effective separation from human subjects, making it easier to create clean and convincing visual effects. The goal is to ensure the "keying" process removes only the background color and not any part of the subject.
