My B1 lower crowns have a grayish color, how do I ensure I get white teeth on the top?

What you are probably seeing is the metal that comprises the substructure of the Porcelain Fused to Metal crowns. That is also what you see when there is a black line or margin at the gum on some crowns. These are done very commonly, but the metal is tough to block out. There is no compound to mix. Porcelain is in powder form and has the consistency of wet sand when it is placed on the metal and fired. I no longer use this technique. I use a machine that mills the crown out of block of hardened porcelain in about 12 minutes using software to design the crown to fit your mouth. I have found these crowns to be much more life like. You might want to express your concerns to your dentist before he does the upper teeth as these are much more visible when we smile and speak. 

You never explained what the color your lower teeth were done in, and what type of material was used (all porcelain, porcelain fused to metal, etc.  There a many different types of materials, crowns and colors.If you want a whiter color, then you might consider a shade in the bleaching range (BL1, BL2 or BL3), and not a plain B1, which is the whitest color porcelain in the natural shade of teeth.  Bleaching shades are whiter than natural color shades.Ideally, you would have both your upper and lower teeth should have the same color and type of porcelain for the best and most esthetic look.This may not be a problem related to the lab, but more a function of the type and color of the crowns that your dentist had the lab do.Sometimes the best option is for the patient to go to the dental lab, and the ceramist can see what the problems are, and work with the dentist in choosing the best material and color.  This is referred to as a "custom shade" and generally adds some cost to the treatment and most labs charge extra for this.You might be interested in the web link below, that discusses tooth color.

I work with a lab that will do a test unit, i.e., they make a single tooth that you can try in on your natural tooth to check the color before completing all of the veneers or crowns. There is a fee for this, my lab charges half of their regular crown lab fee .It is a great way to have a "test drive' of the color. There are underlying colors of natural teeth that can shine through a thin crown or veneer. If this happens with the test unit either the lab or your dentist can use materials to mask out the underlying color and get a better result. Good luck with your new smile!!

Speak to
your dentist about your concerns. He can take pictures of your existing B1
crowns and send them to the lab so that they can ensure your new crowns do not
have that greyish undertone that you are concerned about. Your dentist may
suggest that you go to the lab so that they can evaluate the shade in person,
and this may help to better resolve the issue. You may also want to consider
changing from the B1 shade to the whiter bleach shades