I don't think dithering is your biggest problem here, you should work on the actual shape of column and making sure you know where your light source is, I think it would help if you looked up some reference and had it help you out, dithering is something you do last on whatever you're making, at least how I see it anyway, you want to lay down and ensure the initial shape is good and then ensure the shadows are laid out and general color is there and then you dither and clean up.