To go whiter, you'd have to use some sort of bleaching agent which, I'm guessing, would ruin the shoes. Going darker would be easier.
Maybe you could take them back? Or are they the ideal pair?
Wedding shops may have someone they regularly use. My friend got her shoes at a wedding fair, they were perfect, cheaper than usual and made to the exact colour. There are usually a lot of fairs in the summer with a lot of excellent small businesses offering their services, but it depends how soon your wedding is.
Good luck with the shoes and the big day!