For the Marble map type, you can set or modify the following parameters in the Marble Parameters rollout:
Color # 1 and Color # 2: Click to display the Color Selector to select the two colors for the veins (color 1) and the background (color 2). A third color is generated from colors 1 and 2.
Maps: Click to select or create the bitmaps or procedural maps to appear in the veins or in the background color.
Swap: Switches the position of the two colors.
Size: Sets the spacing between the veins.
Vein Width: Sets the width of the veins.