I just had 6 Emax crowns put on. They cemented them in with dark Grey cement. Is that right to use grey cement? (photo)

It is difficult to say what all is happening in this situation.  I am not aware of any grey cement even existing, let alone being used.  There are many things that could be improved with the photos shared, so being clear on your goals would be step number one.  If color is the ONLY concern, then it may be simple.The gumline is uneven and may require some removal of bone to get it all even.  Once healed, new crowns can be made.  Emax can be cemented or bonded, but I suggest bonding them for added strength, plus the choices in luting resin to help in color enhancement.  Natural teeth are more than a single shade/color, so luting eMax with a colored resin can help make them have natural gradations of color.Most cements do not use a blue light, but bonded resin DOES (but can skip the light if a dual cure activator is added). Some temporary cements use a light to activate, but not all (it sounds as if yours did).  If you are happy with everything, but worried about the materials used, I would simply assume all is well.

Emax porcelain crowns is not for every tooth, it depends on the underlying shade and color of your prepared dentin. Usually if the color of prepared teeth are grayish to dark, it will pick up the color of your underlying tooth, since E-Max crowns are more translucent !The Beauty of Emax crowns are so that it can be bonded AND cemented, Usually with bonded Emax crowns, blue light or white light can be used, for added retentive purposes or possibly brighten the color.In our office, I usually will not use any gray cement, and I do not believe any expert cosmetic dentist will use a gray cement, now if they did, it must have been for shade matching to an adjacent natural tooth. Most cosmetic dentists use clear cement, or possibly to brighten the color a more whiter cement can be used.The gray lines around your front porcelain crowns, could be do cement procedure, with bleeding gum, or exposure of dark root surface !Looking at your pictures, you do show inflamed gums, uneven gum symmetry, in which will require dental treatment to correct.In my opinion the gum architecture should have been taken care of first, and if you had dark colored dentin, another type of crown, possibly lava, zerconia dental crown, or possibly even porcelain fused to gold crown to be used.

I think the crown shapes and contours look alright in the last picture. What catches my eye is the uneven height of your teeth in the other pictures. This is something that can be fixed with cosmetic crown lengthening, and/or braces.The shade difference has been explained well in the other answers

Emax is a material that can be bonded( using the blue light) or cemented( no light). It doesn't appear that the cement has affected the color of your restorations based on your pictures. Technically speaking nothing sounds wrong. If you are happy with the esthetics I don't see a problem based on your description.