While CGI can be used to create films in which all the visuals are created in a digital environment (i.e: “Toy Story”), “Computer Generated Imagery” is also often used in conjunction with live-action footage (i.e: “Jurassic World”… and most Hollywood blockbusters). In that case, the action is shot in front of a bright green (or blue) background which will be replaced by CGI content in the post-production process.
The neon-like color is used because it is not found in skin tone and can thus easily be chroma-keyed (selected and replaced) without affecting the actors. Adobe After Effects is a software popularly used for the chroma-keying process.
“Guardians of the Galaxy” made extensive use of green screens. All the green backgrounds in this video are replaced by CGI content in the final film.
– Anna Pinkas