If the teeth were normal color prior to the veneers, then there are two main possible reasons. 1) the cement used was not an adequate veneer cement. Veneer cements should be light cured not dual cured. Some older dual cured (mix two pastes) can change color. If the veneers are very thin, as most are, this will show through. Light cured cements tend to be more stable. Also, when the veneers are cemented in, the teeth get dehydrated and look whiter. After a few days of rehydration with saliva they will return to normal color. If your teeth were darker prior to the veneers, its possible that they looked good at try-in (dehydrated) but after a few days, they looked darker (rehydrated).
2) the nerve has died from the trauma of the veneer procedure. This can be tested by using a pulp tester or a cotton pellet with cold spray.
Another possibility is improper cementation technique which is filtering particles under the veneers. This goes back to the first reason.
Unfortunately, once cemented, veneers generally need to be cut off and re-made. If you are not happy, I recommend you go back and have them re-done at no charge. This is not your fault since the color change happened after you left the office.
Hope this helps
Dr. T