Before answering this question, are you referring to plastic composite dental veneers or porcelain veneers?
Although modern plastic composite materials that are used today for dental veneers are a huge improvement over materials we would use years ago, they still can discolor and pick up stain with time. A gentle polishing at your cosmetic dentist's office can usually remove any surface stain, but any internal discoloration or color shift of the veneer with plastic composite veneers would be difficult to whiten without redoing the composite veneer completely. This is why so many dentists and patients today greatly prefer porcelain veneers, which are much smoother on the surface and rarely undergo any color change whatsoever or pick up much surface stain. You can continue to drink coffee and smoke, and your porcelain veneers should not change color, and any minute amount of surface stain can usually be removed with gentle brushing, or using a little low abrasive toothpaste on a Q-tip, or returning to your dentist for a gentle polishing.
Dr. Norman Huefner