I have emax veneers. However they are bl1 but are quite grey and dull. Need advice about the shade of cement? (photo)

Correct me if I am wrong but I am assuming these were no to minimal prep veneers and your tooth shade is dulling the high translucent bleach shade Emax veneers.  Your choices would be to have the teeth further prepared for more thickness in the Emax material which will hide the discolored teeth and allow the brightness to shine or to use a more opaque cement that will help shield the natural tooth color.  However, option B is VERY limited in what it can do for you and there is a risk of seeing the line angle of where the tooth ends and the veneer continues with this type of solution.Good luck on your journey!

Going forward, the dentist will likely use a whiter cement. It will likely not be visible like they stated. Another solution would be to change the translucency of the veneers going forward so they appear brighter.

There are different opacities of cement but the difference they make on the final restoration is quite small, and I'd be concerned using a very opaque cement for the very reason you dentist mentioned - you might be able to see a white line where the veneer ends.
A better solution is to use a more opaque material. e.max comes in multiple translucencies, the best solution would be to choose one more opaque and less translucent. If your veneers are made using HT (high translucency) and you want to redo them, I'd chose a more opaque version of e.max like MT (medium translucency).

There are several factors that determine the final appearance and shade. The first factor is , what did your teeth look like before? If they were very discolored and dark, then the emax veneer may not be opaque enough to mask that discoloration.  In cases of very dark natural teeth, the best results generally come with a very experienced creamiest tha can make hand built feldspathic porcelain veneers. The inside of the veneer can have poaquers built into it. That way you don’t have to use a stark opaque cement to bond to your tooth- avoiding the potential for that cement line they are concerned about. 
The other factor is how much preparation was done on your teeth. In darker teeth- reducing a little more can make the veneer thicker, which will naturally let the bleach shade show up brighter.