If your column started off white, apply a base coat of gray paint ofver the entire piece. You can use a mixture of black and white craft paint. If you used the pre-mixed Monster Mud recipe, a base coat color may not be needed. Apply random shades from dark to light over the base coat. (Think about the surface of a multi-colored marble).
Use a sponge or bunched up cheesecloth to gently dab at least two colors over the column surface. Some of the base colors should still show through. Use white and brown to create a pale marble color.
Finally, using a paintbrush with a small tip (1/2 or smaller), drag the same colors (white and brown) in random diagonals starting from the top and moving down. This will create the long streaks in the faux marble. Let dry.
For an extra layer of detail, use a brush to add watered-down, brown craft paint drips. Let the paint run down the large cracks and column sides to add another bit of wear and tear. Faux moss can also be applied.