Random...

Blue

Here's a neat trick in photoshop...

I scan my pencils in in colour. As you can see the blue underdrawing is still visible. I then click on the blue channel in the channels pallette and ta-daaa! the blue is gone. The I convert the image to greyscale discarding all the other channels. Dunno why but it's really satisfying. I then print the pencils out in blue so that I don't have to worry about the inks fading when erasing pencils.