Dark tooth showing through crown? (Photos)

I agree with Dr Sprague. The darkening of an endodontically treated tooth must be masked before cementing a crown. That can mean an opaque crown that doesnt have translucency or an opaque layer applied to the tooth before cementing the crown. 

The tooth discoloration is the result of the underlying tooth structure showing through the porcelain.    The dark tooth needs to be masked out with an opaquer  with direct bonding or placing a coping over the tooth .    Ultimately the ceramist and dentist to develop a new treatment plan and choose a different porcelain choice for the overlying crown. Unfortunately, you will have the start the process over again To get the result you are looking for.  Shape and contour look nice  

Your dentist may need to use a different material for the crown than porcelain. I would recommend a Prismatic Zirconia crown to hide the the underlying shade of the discolored tooth while still having the same superior cosmetic result of a porcelain crown.