Why is my brand new zirconium porcelain crown look dark/gray? (photos)

I recently got a new crown on tooth #7 (it had a root canal done to it before, a lot of the tooth was damaged. After my first crown the finished product looked dark and gray once he demented it. This was an all porcelain crown. I went back and he said we need to red-so it and use zarcomium?? I just got the second crown in a month in a half and it is still all dark and looks really bad. Now he is telling me I need an implant. Does anyone know why the crown keeps turning dark on the whole tooth.