In a large glass bowl, combine baking soda, citric acid, Epsom salt and cornstarch.
Divide mixture into two bowls. Add cosmetic pigment starting with ½ tsp and increasing until desired color is achieved.
Stir together grapeseed oil, water and butterscotch fragrance in a small cup.
Slowly add liquid evenly into each bowl, stirring rapidly to prevent mixture from activating.
Layer bath bomb mixture in molds, alternating bronze and gold. Allow bath bombs to dry overnight before removing.