Prime all areas to be painted with a base coat of white or black. Choose the color depending on whether you want to create a dark or pale marble and whether the color of the stage area can more easily be covered and evened out using white or black. For example, choose black if the existing material features a lot of contrast and dark designs. Let dry.
Paint the area with a coat of the paler shade of gray or beige paint. Let dry, then add a second coat using a circular, swirling paint application; this will create texture that is less obviously painted on as straight brush strokes.
Sponge paint with a kitchen sponge and slightly darker paint to create some spots of texture. Coat the broadest side of the sponge with a light coating of paint and press against the stage. Layer sponge imprints in an irregular pattern over the first coat of paint, leaving about half of the original color still visible and uncovered. Let dry.
Dip the feather in off-white paint and swirl it around in an irregular curvy line pattern to create the veins of the marble. Create lines that branch off from other lines for a more authentic look. Keep the lines as thin as you can.
Spray the entire painted area with varnish to give it the look of polished, smooth marble. Apply at least two coats. Assess the appearance of the stage from the audience area and add more coats, if necessary, to make the marble effect look more realistic.