blue or green, technically makes no difference. The key is lighting, the screen must be evenly lit (try 2 soft lights), then you need to separate the subject by means of back light. Bring the subject forward of the screen so when you light them they do not cast a shadow on the screen (you will need a bigger screen than you think); then you need to back light them. Two 3/4 back lights work the best,ie: behind the subject at about 45 degrees.