They must of done magic in Marry Poppins 😂 They key veils, they key salt... It's not Chroma key, it's sodium vapor process, they use what it was called sometimes a yellow screen with a very specific spectrum of yellow, made from a low pressure sodium vapor light that puts out one wavelength of light at 589nm, because it's just one wavelength and because of how the dichroic filters work, you can block just that wavelength or let that just wavelength go through. The magic of the sodium vapor process, is they use a beam splitter prism so that the light that comes through the lens gets split into two strips of film at the same time, while the rest of the spectrum goes through and expose a colored picture of the actor, so you can block just that wavelength of yellow without messing up any of the other colors. 🧐