Many fragrance oils, have vanillin in its formulation as it has a number of great qualities when it comes to fragrance development.
Vanillin is a compound of the vanilla bean. It's a common ingredients in many scented products. There are many good things about this ingredient however it has the potential to discolour your final products, bath and body products if dosed at a high rate.
It is for this reason we strongly encourage you to undertake small test batches first to determine the stability of your final product.
If the fragrance oil you are using causes discolouration with your products, it's best to reduce the dosing of the fragrance oil as it may contain a high percentage of vanillin.