Can a veneer be placed over zirconium crown?

Discuss your feelings with the dentist. The bridge can be redone to correct the  osmotic issue. It is better to redo the bridge but a porcelain veneer can be done to cement on top of the exsisting bridge. The technique is very technical and works well. 

Sorry to hear about your problems but unfortunately there is nothing that you can do about changing the color once the crown is inserted.  The only option would be to have it redone. Speak to your dentist  about your concerns and maybe they could have a resolution that would satisfy you