How do i paint a white fireplace to look like marble.

Plaster is usually too textured to look like marble. But you can sand the roughed and raised areas or the whole fireplace. When you have a smooth surface, paint it with a white primer. Dry completely between each step. Next, use the lightest cream shade you can paint and a wide foam roller. To make the wall look like one large sheet of marble, roll the cream paint from top left to bottom right in light streaks, like lightning. Do additional streaks above and below the first, but don’t cover all the white. When that has dried, you need a couple of sizes of stiff bristled brushes and paint you want to use for the colored streaks in marble, and three small containers for watered down paint. Blue, pink, and gray are common colors for the marble, but almost any soft color is fine. In one container, pour full strength paint, in another mix water and paint in equal amounts, and in the third mix one part paint to three parts paint. Start with the full strength paint and and the larger brush; dry brush some stripes in a left to right action but at a slant, in short segments divided by a foot or so. While that paint is still wet, use the smaller brush to dry brush more slanted stripe segments, some overlapping parts of the first, and allowing the smears to occur. While that is still wet, use the smaller brush to make more, wetter stripes, overlapping the other stripes in random order. Sponge up drips although some spots are okay. When all of the paint is completely dry, use a semigloss clear sealer to protect the painting and look like polished marble. Best wishes ☺️

There are kits for this exact purpose. I saw them in Home Depot. Everything you will need is in the kit.