For green marble, start with a black base coat.
The key to marble is to recreate the color variations that you see in the natural stone as well as the veining that gives marble it's distinctive look.
To get the variation dip a damp sea sponge into each of the colors and dab it onto your project. Continue adding paint making sure that your colors vary in depth and shade just like on the natural stone.
A piece of scrap paper on the side is a handy spot to dab and mix the colors. A clean cloth will help dab away excess paint from the project. allow the paint to dry completely.
To add the veining you can use a detail brush or a feather. Use your lightest shade of paint mixed with some white or gray. Dip the brush or feather into some water and then some paint.
Then using a light hand work veining lines over the surface of the paint branching off from your main lines with lighter paint.
Once the project is completely dry add some polyurethane so it has that look of real marble. I'm Mrs. Fixit and it's just that simple!