Why in the world would you ever want to shoot a sports portrait session on a blue or green screen? Lots of reasons. For beginners you might want to create an awesome custom background for your portraits. Your shoot might include creating a panoramic, sports banner, or season schedule product featuring the team and shooting on blue or green screen will make your Photoshop extractions a snap. Regardless of what the reasons, you must be equipped and prepared to shoot on the blue or green screen. Here are a few suggested tips that have helped me when using blue or green screen....
Know the color uniforms being worn by the team and plan accordingly. Don't use a green screen if the uniform contains green and similarly don't use a blue screen if the team uniforms contain blue. If you have teams containing both colors go with the screen that will be most beneficial to extracting your subjects.
Light the blue or green screen background as evenly as possible. I use one Alien Bee strobe with a bouncing reflecting umbrella on each side of the backdrop to reflect light onto the backdrop. Using a light meter, I measure the light falling on the center of the backdrop equal to or slightly greater than my main light on my subject's highlight side. I typically shoot my subjects at F11 @125/sec ISO 200 for the highlight side.
Use a blue or green screen that reduces light bouncing back toward the subject. I use foam backed blue/green screens that do very well at reducing any bounce back. The result of getting color bouncing back is that your subject's skin, clothing, hair can come contaminated with excessive amounts of color from your backdrop and make it very difficult to get a clean extraction in post.
Purchase a plugin to help with your extractions in Photoshop. I use the Primatte plugin from Digital Anarchy for all my extractions. This tool has saved me hundreds of hours and is worth every dollar spent on it.
That's about it. If you're new to this type of shooting, do some testing before your photoshoot and get comfortable with the setup and post processing of the images.