Can a veneer be placed on top of a crown?

There should not be much of a change in the shade of the crown. You are mentioning days and not months or years. A crown like an E Max can look just as good as a veneer. I would speak with your dentist and discuss you are unhappy with the color and see what he says. You will not want to place a veneer on top of the crown. Basically you would need to remove the crown and place the veneer and now you are paying for two procedures. Makes no sense! Speak to your dentist again!

My guess is that your dentist placed a porcelain-fused-to-metal crown, and not an all-ceramic crown.No, although a veneer can "technically" be placed over an existing crown, it is better to replace the crown for a number of reasons, the least being it would cost about the same amount to do a new all ceramic crown as it would to place a veneer on top of the old crown. I highly recommend not trying to do that. You will be disappointed.It's not that veneers match tooth color better than crowns, it is that ALL-CERAMIC restorations, whether they are veneers or crowns, match the existing teeth better than porcelain fused to metal crowns.

If you are unhappy about the aesthetics of your crown there are options to improve the result. In response to your question, a veneer can be bonded on a crown but typically this is a very advanced technique, and probably is not needed for your case. Crowns generally can be made to have equal aesthetics and shade match as veneers if the proper material and shading is used. It is difficult to say without seeing photos, but most likely the easiest solution is to simply redo the crown. The dentist should send pictures to the dental laboratory and shade-map your tooth to account for any variations in shades, translucencies, craze lines, fluorosis spots, etc. and precisely blend it with the adjacent teeth. Depending on the material the crown is made of, the dentist may want to consider switching to a more aesthetic material. In general, "layered lithium disilicate crowns" are the most natural, but there may be certain situations where this material cannot be used.