Crowns & restorative work can acquire stain which can be removed & controlled, but the actually color of it is processed at the lab and that color cannot be changed.
Your dentist has done his best to match it to the shade of your teeth. Some teeth have a natural yellowish or grayish color, and your dentist tells the lab tech to match the porcelain or ceramic to whatever your natural color characteristics are. You can certainly use a whitening toothpaste or mouthwash to help keep staining down, but using something like crest white strips or bleaching methods that have peroxide or carboxide in it would not be recommended. If you do use something like this, you are going to make all of your natural teeth whiter, yet your implant is going to stay whatever color the porcelain is. I am not aware of any permanent restoration (crown, implant crown, bridge work, fillings) that can be "bleached."
My sister has implants she uses a whitener all the time. It is kind of hard to whiten your teeth and not have one touched. For example there are tooth paste with whiteners you brush your teeth with it and that tooth is being whitened with the paste.
The only thing that you can't do to that tooth is get it bleached only the dentist can do bleaching and he knows what to do with the tooth. Your best best is to brush with backing soda and peroxide that is what all these tooth paste use in order to help whiten teeth or just got to the dentist and get your teeth cleaned that's all.
So why the heck did my dentist put in a product that cannot be whitened?
As far as I know, you cannot bleach any kind of crown or restorative work.