It's normal that the teeth can't get nutrition supply after the nerve of the teeth is destroyed, and it's certain that the teeth will turn yellow and black slowly. It is normal that the color of porcelain teeth is different from that of your own teeth. Even if the color numbers are very close, it can be seen that the porcelain teeth are darker in the darker places. If the color difference is too big, you have to ask the doctor to change your color number again, which is closer to your own teeth, but the process is more painful.