I used to use a green bed sheet, IT worked, I was amazed at how much the real deal worked when I go one though. Finding a green enough green, or a light enough blue that is still a true blue in bed sheets can be a trick, but it does work.
You can buy green paper green screens to. They are more permanent and are about 1/3rd the cost. They are almost a thin cardboard and work wonderfully. But are not vary portable and are not as durable. Many professional studios with more permanent stages use these paper screens.
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You might also look at your local fabric store and see how the fabric their compares price wise as you will more than likely be able to finned a suitable colored fabric.Â