Depending what tool you're using to draw the line to the screen will change the solution to this question, but the simplest method would be to have the intensity of line color be a function of its distance to the center of the line. Therefore you would have a black line that greys out at the edges.
The other form that you could be talking about was the fact that paper is not perfectly smooth, thus the pencil will be darker on the hills and lighter on the valleys (on a minute scale of course). To do this you could just apply a couple of layers of noise in different shades of grey over your black pencil lines.
The are further methods that come to mind, but they all depend on how you're actually planning on drawing to screen.
For further research you can look into how digital artists fake pencil effects and see if any of those methods translate well into a programmable method.