A chroma-key is the device that changes the "green screen" in the studio into an image.
That image can be a computer generated graphic or a camera image.
Any image the meteorologist creates on the computer will appear behind the meteorologist rather than the "green screen" for the television viewers.
Unfortunately, the forecasters are not able to wear clothing with the same color green (used on the wall). If they did wear green, it would appear as a large hole on their body.