Answers:
If it is a porcelain crown over you intuitive tooth you cannot silver the color as it is made surrounded by a lab and is everlasting. You can however acquire it polished to remove any stain that might be building up on it. If it is the inherent crown (part of the tooth within the mouth) that you are referring to afterwards you can use a whitening agent (like Crest Whitestrips) on them.
Hope that help!
JAMRDH -a dental hygienist
if it's a crown, after you can't generate them whiter because the color of it be made to contest the rest of your teeth at the time your have it placed.