Add baking soda, citric acid, cornstarch, and ¼ cup of the Epsom salts (set aside the remaining ¼ cup of Epsom salts) to a large bowl and stir well with the whisk.
Add the sweet almond oil and water to a jar with a lid and shake well.
Very slowly add the liquid to the dry mixture, while whisking the entire time.
Take out ⅓ cup of the mixture and add it to a smaller bowl.
Add the silver-grey mica to the mixture in the larger bowl and stir until the color is solid.
Fill one bath bomb mold halfway with the mixture, then press around the sides, until the entire mold is coated in the silver. Repeat for the rest of the molds.
Add a spoonful of the non-colored mixture into the middle, then press down, making sure that you can still see the silver-grey edges.
Allow bath bombs to set for a day, then unmold by gently pushing from one side until they release.
Add the purple mica powder to the Epsom salts that you had set aside, and stir until the color is solid.
Add a small spoonful of Epsom salts to the center of each bath bomb, making sure that you can still see the edges of the silver-grey and the white.
Store upright in a flat container.