The only option is to redo the veneers. Some veneers can darken is certain types of adhesives were used to put them on. They unfortunatly will need to be removed and remade.
If your veneers are made of porcelain it won't be possible to change the color without replacing them. However, if your veneers are made of composite or resin you may be able to lighten and remove superficial stains with different whitening products on the market and or having them polished professionally.