But surely they had different bluescreens for different sets?
TOURGUIDE: Over here you can see the bluescreen used to shoot all of the senate scenes...
GROOP: Oooohhhhh....
TG: And here you can see the Mustafar 'red screen'.
GROOP: Ahhhhhh...
TG: And, best of all, the Naboobian green screen!
GROOP: <sound of heads asploding>