Should Temporary Veneers turn bluish-gray?

Temps are a great way to check size shape and color of veneers. They need to be relatively easy to remove and leave no residue behind to interfere with bonding your permanent veneers. A change in color will not likely be a factor in the color of the final veneers unless it's the actual tooth that is changing color. And that is unlikely. 

Hi,
You should not worry about temp veneers, but before they place your permanent veneers make sure you like them and feel comfortable with them.

its common , no need to worry about temp veneers. but before permanent veneer cementation check veneer texture and colour. because after cementation changers are not possible.

Sometimes, despite valiant efforts from the dentist, temporary veneers "leak".  Saliva can get underneath and discolor, making the veneers appear grey or blue, usually along the gumline.  It is nothing to be worried about, when the temporary veneers are removed, some hydrogen peroxide can be placed and rinsed, and the discoloration is completely gone (and the tooth completely disinfected).