on Thursday, 7 December 2017
I have found very little difference between the Grey 9 and the Grey 3 Liquid pencil when using them, but the supplier does use the different numbers to distinguish that the graphite is sourced from different areas of the world.
Here is the information for the supplier web site.
The graphite powder used in Liquid Pencil Grey 3 is sourced from China and has a partially formed structure that contains additional compounds, which give it a deep black colour with a blue undertone. In contrast, the graphite used in Liquid Pencil Grey 9 comes from Sri Lanka and has a more uniform chemical structure that imparts a silver metallic sheen to the finish.
I hope this helps,