granpam, good point on etching. i forgot to mention that i do that ! (when i paint with vodka and brown color)
for marbeling...you take two shades of fondant (brown/white...dark brown / light brown etc) and you roll a long "sausage" out of each color. twist them together like a rope. fold in half, twist and pull a few times until you get an exagerated marble look. keep in mind that when you flatten it to roll it out, the grain will widen. so, depending on the grain size that you want, will depend on how many sausages you roll out and twist together. you dont want to over mix/twist it because then it wont really look like woodgrain when you roll it out, it will just kind of look like a mess and not very distinct.
now, if you want to paint the bat, cover the cake in fondant (im assuming the cake is bat shaped? or is this a fondant accent to a cake?) you can use white, or light brown depending on the deep shade of brown you want. use brown gel/paste color and a few drops of PURE vanilla, orange, lemon or mint extract, vodka or everclear until you get a consistency that paints on semi transparent. what happens is,when you brush the color on the fondant, then backstroke again, it smudges the "paint" that has begun to dry (because of the alcohol content evaporating quickly) and gives a grain effect.
its quite cool actually. you can add knots and nail holes (though in this case you dont want planks or nail holes)....
have fun,post a pic when youre done!