Blue screen or Green screen?

There has been a long debate from filmmakers and visual effects people about the use of blue and green screen. Is blue screen better than green screen, or the other way around?
To understand this debate and pick the right colour for your blue/green screen work, we'll first need to understand the process of this technique. So let's dive into the world of keying and compositing.

Chroma keying, as the technique is officially called was created already back in 1933, but really gained it's full purpose during the making of the Star Wars movie back in 1976. The idea of this technique is to seperate part of an image you do want in the shot from the part that you don't want in the shot (the blue or green screen) in order to replace it with something else using visual effects. But since the use of film compositing, and later the use of digital compositing don't understand what the eyes are seeing, they needed to make it clear to the film or digital compositing system what they wanted in the shot, and what had to be deleted. In order to make it clear they decided the best way to do this, was by using a strong contrasting colour. Selecting a colour as a background, completelt different from the foreground object which we do want in the shot, allowed the compositing system to easily erase the background colour, only leaving the foreground object in the shot.

Since the idea is that you use a contrasting colour, that has to contrast heavily with that which you want to leave in the sho, I guess you could come up with lots of arguements about whichcolour is better and why. Some people claim that when shooting digital green screen is better than blue screen, others claim it has to do with the skin colour that would contrast better against a certain colour. In the original Star Wars movie they used blue screen. This decision was made because of the colours of the models. And in the end, that's what it comes down to.

There's no real better colour when it comes to keying. It all just depends on what contrasts the best. Think about the colour of cloths, if people are wearing jeans, blue is going to be a dominant colour in a shot, and perhaps green would contrast better with that. And when you're shooting a forrest or plants, perhaps blue would be a better pick. My advice to any filmmaker would be to always involve the visual effects supervisor, or someone who has a good knowledge of visual effects, to consult on these things. Let him be there on set with you when you're shootin it, so he can help you out with what would give the best result.

Most important with blue or green screen is that the blue or green screen needs to be as flat as possible. Shadows on the green or blue screen break the even colour, and therefore create a contrast in the the contrast you're already trying to create. So try to avoid wrinkles in your green screen, and try to light it as even as you can. What also helps your end result is avoiding spill. Spill is what visual effects people refer to when the green or blue colour from the blue or green screen, gets reflected on the object or person you're trying to shoot. The picture below displays an example of green spill on one side of the actors face, which will make it more difficult to remove the green background, without erasing a part of her face.

The best way to prevent spill on what you're trying to shoot, is to keep the blue or green screen only on places where we actually need them, and keep it behind the object or person you are shooting. So don't put it to one side. Of course this will not be always a possibility, and it is possible to do it with spill, but the result without spill will always be better.