Paint the screen area with water-based primer.
Paint over the primer with a light gray, satin, water-based paint. Gray will provide better contrast than white. A satin finish will provide the correct balance of light absorption and light reflection. A matte finish would absorb too much light and a gloss finish would reflect too much light.
Spray paint the screen with a light misting of silver metallic spray paint. Allow the spray paint to dry.
Spray paint the screen one more time with a light misting of silver metallic spray paint to even out the coverage. The screen should not be totally covered in the spray paint---just speckled enough to provide subtle enhanced luminance when an image is projected. Too much will cause the image to be overly reflected.
Paint a 4 inch border around the screen with matte black paint to create a crisp border around projected images.