Thought I'd try again ...
Have there been any improvements over the past 5 or 6 years in PFM crown
shading, particularly when matching them to yellowy teeth?

Also, when all-porcelain or all-ceramic crowns are used on bicuspids and
molars, how does their longevity compare with that of PFMs?

Thanks!

There have been improvements in place since the late 80's that have made it
possible to make PFM crowns that look awesomely good. The process to make a
PFM crown is 100% handicraft from bulk material. So if you contract the same
craftsmen you did 5 or 6 years ago, the question is if they have changed
their approach.

Lars

Would this be true of crowns for people who have yellow teeth? We seem to have
a harder time getting undetectable crowns than people with whiter teeth.

Yes dear to say it can be done. If the technician has the opportunity to
examine, color - translucence - structure of the teeth's they are to
imitate, in first hand. This will greatly improve the prospect of a good
blend then the adequately educated technician have the in depth knowledge in
the nuture of the materials used. But the common procedure is that the
information is filtered through the dentist and passed on to the technician
via standardized shade tabs. The latter, more tortuous path will of course
enhance the possibility for misinformation. Even in larger metropolitan
areas it seams to be a flavor among dentists to have the lab located
elsewhere (mail-order). I don't know why, I could only guess and speculate.

As a subordinate clause I could mention that it's rare in the world that
there is obligation or requirements that dental labs have to have educated
technicians aboard. Trough out the history, in Sweden, have all struggle to
build a standardized and recognized education been counteracted by the
private dentistry associations. But despite that it have been done.

Lars