HI!!! For a fast, simple solution, you might simply try adding white modeling chocolate to your white fondant. Do so after each is made…just kneed one into the other (start off with 1:1, then adjust if necessary). I think this will work just fine, and most good cake books (the Cake Bible included) will have recipes for the white modeling chocolate.
OR you can order white chocolate fondant and save yourself some time! The consistency of pre-made fondant is often much better than homemade. I would recommend opting for a good-quality fondant, though…some of them can be pricey.