My veneers are too white, if a dark cement was added to the back would that change the color and make them less bright?

Hi:it is diffcult to tell if the cement could have changed the color.  Depending on the material of the porcelain and the level of translucency, it could have not made any difference.Best,Dr. Maddahi

Once porcelain veneers are bonded in place, there is no way to alter the color except to remove them. Were you given a chance to see them in place and approve the appearance before they were bonded? This is something I do with every single crown I place, but especially so with smile changes or veneers. In fact, we also take photos of the crowns or veneers in place before bonding and show the photos to the patient before bonding, as looking in the mirror vs. looking at a photograph of yourself gives VERY different impressions! In some cases when making a significant color change from natural teeth to veneers, the new color, although lovely and bright, can take some time getting used to! If you've just had the veneers a few days, give yourself a few weeks to accommodate. I can speak personally to this, as I felt it myself when I had my smile "done", but now I feel like this is the smile I was MEANT to have from birth. I would recommend contacting your dentist to share your concerns. If after several weeks you still feel the veneers are too bright, unfortunately the only remedy is to have them removed and replaced. Good luck!

Thank you for your question. Unfortunately that is not likely to be possible for two reasons:
The extra space that would be required behind the teeth would mean that they would not fit together properly in the correct biteIf there was a little space available, material added to the back would change the opacity of the tooth (how much light that passes through it) which would make them look slightly dead or lifeless. 
Really, you need to speak to a cosmetic dentist to work on a new veneers that better match your needs. A good dentist would have less you had a mock up of your teeth, so you could 'try out' the shade beforehand. 
Hope you have a better outcome this time - 

Kind regards
Thomas Dey, BDS, MSc