Hello, catst! Thanks for asking! Unfortunately, we do not have the gray color; however, we have a gray/brown mix https://www.houzz.com/products/thin-brick-veneer-case-of-4305-sq-ft-4m2-prvw-vr~45283318?lid=32174514 

or with a different structure - gray/white/black one https://www.houzz.com/products/manufactured-stone-veneer-case-of-4305-sq-ft-4m2-prvw-vr~45272682?lid=32164670 

Please let us know if you would like to see the samples!

P.S: you can use even a blue mortar! Absolutely, yes! We glue them on a regular thin set adhesive and it's eventually gray, add any pigments you like and here we go! Good luck with your beautiful home!