Your fireplace was made to resemble stone, so I suggest you look at stone tile samples, bringing some home to see what color looks good with other things in the room. Then you can either hire a decorative painter to paint the fireplace to look like the stone tile sample, or you can attempt to do it yourself. The finish should be a matte finish, not shiny. If you try to do it yourself, you might try to practice on cardboard or paper first. You'll probably need two colors or more for creating a realistic stone look.

Since it was made to look like it was carved out of stone, do not try to use totally different colors on different parts of it. You may want to first paint it a taupe color and then apply a slightly different color with a rag, dabbing the color and smearing it about. Watering down the paint may be helpful. Have fun!