The importance of the stump shade.

This patient had a green tooth!  I guess it is a stain from some oxide in the old silver filling.

Regardless, if an all ceramic crown is prescribed then the stump shade needs to be communicated to the technician.  The stump shade will definitely influence the final shade of the crown, even if an opaque ceramic is used.
Cement shades will also influence the final look of the all ceramic crown greatly, as well.  Often, an anterior single crown can look more harmonious just by using a higher or lower value luting cement.