I have heard of staining crowns in the mouth, but with ceramic crowns
this generally will only be a surface stain that will wear away in short
order.  In the old days with acrylic veneers this was more feasible
(although not always pretty).
    I suppose it would be possible to dress down the surface, etch and put
a different shade veneer over a crown, but it's probably just as easy to
redo the crown.  Occasionally you can pop a crown off and send to a lab
and have the porcelain refired.  However, sometimes this is risky for
the tooth under the crown, so the dentist should consider the potential
for damage before trying this.
    I've seen some TV personalities whose dentists seem to have used the
Kohler or American Standard shadeguide--Regis Philbin, Morton Downey Jr.
and Randy Jackson are three that come to mind.  It's difficult to
believe that these guys don't or didn't know that their bridges look
fake, so I'm assuming that this is what they were aiming for.

Steve

I am 90% sure there is not way to change the color of your
crown/bridge.  Porcelain color are mixed and then baked in high heat.
There is no effective adhesive to glue to veneer to porcelain.  It can
be done but how long will it stay is the question.

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