Recent studies show that bleaching gels will penetrate teeth from behind. So if you have porcelain veneers on the front of the teeth, you can whiten the part of the tooth under the veneer. But the porcelain will not whiten. The result could be some very slight lightening of the color, depending on how translucent the porcelain is.
Your situation shows the importance of having your porcelain veneers done by an expert cosmetic dentist. These cosmetic dentists, who are passionate about appearance, will always suggest bleaching or whitening your teeth before they place porcelain veneers. Then they will match the porcelain veneers to your new whiter shade. They also know how to have porcelain veneers made to the degree of whiteness you desire, which general dentists often don’t know how to do.