Yes, it is possible that you are noticing a real change in the overall shade of your veneers, although porcelain veneers themselves are generally very resistant to staining and are considered highly color-stable. With porcelain veneers, the final color that you see is actually a combination of the porcelain, the bonding cement underneath it, and the underlying natural tooth. Over time, subtle changes in the resin cement or underlying tooth can sometimes influence the appearance of the veneer, particularly when the porcelain is relatively thin or translucent. There is also another factor that I see fairly commonly: patients can simply become accustomed to the brightness of their new smile. A shade that initially looked dramatically whiter may no longer seem quite as bright six months later because your eye has adjusted to it. That does not necessarily mean this is what happened in your case, but it is worth considering. Since you selected approximately a B2 shade, there are certainly significantly brighter veneer shades available if your goal is a whiter smile. The larger question is whether the difference in color bothers you enough to justify replacing veneers that are only six months old. Redoing porcelain veneers is a significant procedure, and you already know from having gone through it once how much is involved. It is also generally preferable to avoid repeatedly removing and replacing veneers unless there is a meaningful reason to do so, because every replacement has the potential to require additional alteration of the underlying tooth structure. Before making that decision, I would first have the veneers professionally cleaned and evaluated by your cosmetic dentist to make sure there is not any surface staining, bonding material, or other correctable reason for the change you are seeing. If they still appear too dark afterward, then the decision becomes very personal. If going substantially whiter would make enough of a difference in how you feel about your smile to justify going through the veneer process again, replacement is certainly something that can be discussed. If you do redo them, I would spend considerable time selecting the new shade beforehand so you are confident the brighter color is what you truly want long-term.