In order to create a pencil line, you must choose a natural powder colorant for your soap. Examples are turmeric powder, cocoa powder and annatto powder.
First is once you have placed your first layer, get a tablespoon of the powder and sift the powder all throughout the top of the soap batter until all the batter is sprinkled with it. Then pour your second layer over the sprinkled top of the first layer.
Leave to harden. Unmold. Cure. Use. :)