Combine baking soda citric acid, Epsom salt, cornstarch and pink cosmetic pigment powder in a large bowl.
In a small cup, mix grapeseed oil, fragrance oil and water.
Slowly pour liquid ingredients into dry mixture, quickly whisking ingredients together to prevent the mixture from fizzing too much. Test mixture by squeezing a handful to see if it holds its shape (see video above). If the mixture is too dry, lightly spray witch hazel until it can hold its shape.
Firmly press bath bomb mixture into mold. Allow to dry overnight.
Carefully remove bath bombs from mold.
Mix together ¼ tsp cosmetic powder with a few drops of isopropyl alcohol, use a small brush to paint the center of the bath bomb.