Can crowns darken? A few years ago I had my teeth bleached and I had crowns placed on my four front teeth to match.

Modern crowns are made from a highly dense material that is stain resistant. If there is discoloration there are 2 main possibilities that exist:1. Internal (Pulp-related) discoloration. Sometimes the pulp (nerve) of the tooth can die, discolor and stain the remaining tooth structure. modern crowns can be very translucent, so the underlying tooth structure can show through.2. External. The crown can have the outside layer rubbed off from external mechanical (extremely hard brushing, etc) or chemically (acid in the form of soda beverages, lemons, etc.). Once the glaze layer is removed the crown is more susceptible to staining.Thanks! Hope that helps.

Older porcelain crowns typically would yellow over time as the pigments in the porcelain broke down.  Newer porcelain has an improved composition that will keep the porcelain color stable.   If your crowns are only a few years old this would be extremely odd for them to discolor as you described.  It is not unusual for crowns to get external staining from certain foods, drinks, smoking, etc. which can be polished off during routine cleanings.  Using whitening products will not work on your crowns.  If the crowns were not properly polished and glazed this could cause discoloration.  Also, there is an increasing amount of dentists who send their lab work to China to decrease lab fees, these products may not be as regulated and be of questionable quality. I would definitely discuss this with your dentist.  They should have the original shade of the crowns and be able to hold that shade up to compare to the existing crowns to tell if there has been a shade change or staining. 

Crowns can stain with time, however their level of staining is usually less significant vs natural teeth. If the crowns show staining and getting darker in color vs their original color, they can be lightened back to their original color. In other words crowns can not get any whiter in their color than what they used to be when they were originally made. However if they have gotten darker with time, then whitening the teeth can often bring back the original color of the crowns.Please consult with your dental professional about the different levels of whitening that may be available for your specific needs.All the best.