Add the baking soda, epsom salts, citric acid, and cornstarch to a large bowl, and use a whisk to stir the ingredients until they are fully combined.
Add the sweet almond oil and water to a small jar with a lid and shake well.
Very slowly add the liquid mixture to the dry mixture, while whisking the entire time, so you do not activate the fizz. Stir well.
Divide the mixture evenly between 6 small bowls.
Add one color of mica powder to each bowl, and stir well until the color is solid.
Begin layering colors into the bath bomb molds. Red, yellow, then orange in one half; green, blue, then purple in the other half. Make sure to overfill the mold with the third color.
Press the two halves together, and lightly twist.
Allow bath bombs to set in the molds for 4-6 hours, then you may remove them to finish setting.
Store in an airtight container or bag.