The final esthetics of an anterior all porcelain restoration that does not contain an opaque core, is the result of the color and light properties of the tooth combined with the restoration. Because of this a critical piece of esthetic information for the lab is the stump shade and a stump shade photograph. Different technicians have preferences about taking a stump shade. There are special shade guides that you can purchase for taking a stump shade. One of the most common is the Natural Die Material Shade Guide from Ivoclar. The shade nomenclature uses ND and then a number from one to nine. One of the advantages of this system is the technician can actually fabricate the die from the stone that matches the stump shade you chose. This better allows the technician to see a similar color and light effect that you will see with the restoration int he mouth. Another way to approach this is to use your traditional shade guide and take a separate shade for the stump. For the final restoration you may be asking for a B1 body with 0M1 at the incisal one-third on a prep that has a stump shade of A2.
In addition to the photographs I take pre-operatively for the technician to show final shade information. I also photograph the prep along with shade tabs to show the stump shade. I put multiple tabs in the holder, the one that matches, one that has more chroma and one that has less chroma, from the same hue. With retractors in place, have your assistant hold the tabs incisal edge to incisal edge with the prep for your photograph. If you are using a restoration that has a core and the intent is to mask out the color of the prep completely, than taking a stump shade adds no value.