The first two will be base coats, so select from same family, preferably off-white. Using a large sea sponge, apply a light base coat in one of your off-white colors and let dry. Then, use your other off-white to apply another sponged coat over top of the first. Some of first application to show through. Use a feather and accent color to create the marble veins.
Goose feathers work great.
Mix one part paint conditioner with three parts accent color. The accent color might be a hue that works with accessories and fabrics. Dip the edge of the feather into the paint and drag, push or roll it across the surface. Shift the feather's position as you apply it to make interesting patterns. Do large veins, then cross over with finer veins.