Any key color can work, as long as it’s distinct from the subject. Green is popular because it’s uncommon in skin tones and many subjects, and in digital cameras it’s especially practical because sensors typically record more green information than red or blue, which can help produce a cleaner key. Green also tends to cause less noticeable color spill than the older blue-screen approach.

Black is different: it’s usually not a chroma key in the usual sense, but more like a luma key based on brightness. Its main advantage is that it can be created easily by lighting the subject well while keeping light off the background, causing the background to fall to black.

The downside is that black is much less forgiving. Any dark clothing, hair, or deep shadows on the subject can blend into the background, making separation difficult. So black is useful when you can tightly control lighting and keep the subject clearly brighter than the background, but it’s generally less versatile than green for standard keying.