Whitening my front 6 Crowns, how do labs accomplish this on something that is supposed to be color stable?

Depending on what type of crowns you are having made, the laboratory can make minor changes to the color and shape of the porcelain. Most cosmetic dentists will only place all ceramic crowns on front teeth. These crowns are made a couple of ways. 
The first and most popular method is called pressable ceramics and Emax is the standard material for these crowns. Usually the porcelain is made a lighter shade and then staining is added to give the crown a more natural appearance. If your looking for a whiter shade, the staining can be removed which will make the crowns look whiter. 
The pressable ceramic crowns are also made another way by layering the porcelain in a process called a cut back technique. These crowns are first pressed and then porcelain is stacked on top to give more depth and natural appearance. The crowns are more natural looking than the ones that are stained. If you feel that the crowns should be whiter, the lab can remove some of the stacked porcelain and re-stack it with a lighter shade.

Normally the lab would remove the surface layer of the porcelain and replace it with a lighter shade porcelain. This is an easy and common lab procedure. 

That's a great question. The only way to whiten already made crowns is to remove porcelain from the crowns and replace it with a lighter shade of porcelain. There is no magic crown whitening formula.