Choose a color that is NOT prominent in your subject. For example, if the subject has green clothing on, choose the Blue Chroma key Screen / Backdrop. If the subject has blue clothing, you would then choose the Green Chroma key Screen / Background. Many photographers have both a Blue and Green Chroma key backdrop for this reason.

So if your subject is wearing green – use a blue screen! If your subject is wearing blue – use a green screen! Unless of course you are looking to make special effects (like an invisible man movie) where your subject disappears into the background.