Hi,
The cap that turned brown is may be because of the metal base of the cap that is generally called as PFM (PORCELAIN FUSED TO METAL). Porcelain is the tooth colour material that is fused on metal base to give strength. there are materials now that do not need metal base. they do not have discoloration issues. There is nothing to worry about the previous work done. that was done quite well. now when you want to go for its correction, go ahead without any fear.
Next Steps
My advise is that now you go to the dentist who is MDS (Conservative Dentistry & Endodontist). He will do RCT (Root Canal Treatment) first because it is required now. He will explain you why you need this procedure. Then he will give you ZIRCONIUM cap. Your smile and your confidence will be boosted back soon. ALL THE BEST. Regards - Dr. Ankit Upadhayaya, URBAN SMILES, GALLERIA MARKET, GURGAON