The most obvious reason is color contrast. Blue and green are typically far removed from the natural skin tones, making them ideal for chroma keying. When a subject is dressed in clothing that closely matches their skin tone, such as hands or faces, unintentional keying can occur, making the final composite appear incorrect. For instance, red tones are visible when not properly chroma keyed, as green is the easiest to key out, followed by blue, and ultimately red is the last.

Lighting and Exposure

Additionally, blue and green are excellent choices for lighting and exposure. Both colors can be well-lit and provide a uniform surface that is easy to key out. Green screens in particular, are highly reflective and can capture light effectively, leading to a cleaner and more vibrant key. This is crucial in video production, as a poorly lit or uneven background can lead to a messier final output.

Digital Sensors

Much of the digital camera technology leans towards sensitivity to green light. Most digital cameras are more sensitive to green, which means that green screens can produce a cleaner and more vibrant key. When a camera captures more detail in the green spectrum, it enhances the overall quality of the composite, making it easier to edit and enhance the final product.

Post-Production Flexibility

Post-production is a critical phase in video production, and green screens often provide less noise and fewer artifacts compared to blue. This is particularly advantageous when layering images, as it results in a cleaner composite with fewer distractions. Historically, blue screens were used earlier in the film industry, but as technology advanced, green screens became more prevalent due to their superior advantages in keying and post-production.

Historical Use

The use of blue and green screens in video production has deep historical roots. Blue screens were once the standard, but as technology evolved, green screens became more popular. This shift is largely due to the reasons mentioned above, including their ability to produce cleaner keys, greater sensitivity to green light, and better post-production flexibility. While other colors can be used for chroma keying, they are generally less practical for the reasons outlined.

Other Color Considerations

While blue and green are the most commonly used colors, in theory, any color can be keyed out. In a unique anecdote from my high school days, I recall using a Panasonic video switchboard for morning announcements. During a session in the studio, I noticed a chroma key knob/dial with a color spectrum ring around it. The system allowed for any color to be keyed out, not just red, green, or blue. This broad spectrum capability highlights the flexibility of chroma key systems and underscores why blue and green have remained the most practical and widely used choices for keying.

Conclusion: The choice to use blue and green for chroma keying is well-grounded in practical considerations. From color contrast to digital sensor sensitivity, these colors offer the best results for creating seamless and professional composites. Understanding the reasons behind this choice can help video producers make informed decisions when setting up their production environments.