I went to a Chroma-key workshop at the ACMI in Melbourne this afternoon which was pretty interesting (made a 6 minute film using just a green screen).

With regards to screen colour the original "blue screen" was used because the human skin gives off very little blue light spectrum and it worked best for film cameras. However with digital technology, green screening was found to be better for general use.

However you can use any colour theoretically and many studios use different colours for specific applications (e.g. RED screening for close up shots of models). We were told today to pick a colour to suit your need (e.g. if your character is wearing a green costume, green screening is going to cause you problems.

Andy