Combine baking soda, citric acid, epsom salt and cornstarch in a large bowl.
Divide mixture into two bowls. Add cosmetic pigment starting with ½ tsp and increasing in small increments until desired color is achieved. Note: the dry mixture will look lighter than the end result. You can use GEL food coloring in exchange for the cosmetic pigment (use gloves to fully incorporate into the mixture).
In a small bowl, combine grapeseed oil, water and peony fragrance.
Slowly add fragrance into both mixture bowls; stirring rapidly while pouring to prevent the mixture from fizzing.
Press mixture into bath bomb molds in alternating colors. Allow to dry overnight before removing from mold.