Yeah but what about when the Goldfish is online?

If you need to erase a bunch of the image, to replace it with something else, it's easier to do if the parts you want to get rid of are a bright, distinct color.

So if you're making a Marvel movie and your actor is supposed to be on an alien planet, but you're going to add the planet later, you film the actor in front of a bright green background. Then you can take out all the green and you have your actor on their own and you can drop them into the alien planet that you created in the computer afterwards.

Green is the most common color because it is far away from human skin, and not a color we normally wear. You will sometimes see blue, or even purple used instead depending on what is being filmed.