Hi, Luca,
Yeah, it sounds like you used a fragrance oil that contains vanilla. Vanilla will turn your soap brown unless you use a vanilla color stabilizer. Bramble Berry sells one:
http://www.brambleberry.com/Vanilla-Col ... P4156.aspxStabilizer keeps your soap looking white, but keep in mind that if the fragrance oil contains a lot of vanilla, it may still discolor some. For example, in my experience the Buttercream and Snickerdoodle FO turns a creamy beige after a few weeks even with VCS. It would have discolored to a much greater extent without the VCS, however.
Usually, the vendor will give you a heads-up if the fragrance oil contains vanilla or discolors. If it discolors tan to dark brown, it's probably got vanilla in it. I use the vanilla color stabilizer in equal proportions to the fragrance oil, and I mix it with the FO in a little bowl before adding it to my base.
This Soap Queen TV video is very helpful for learning how to use the vanilla color stabilizer:
http://www.youtube.com/user/soapqueentv ... -KjcS-L2SgHope that helps!