In a large bowl, whisk together cornstarch, citric acid, salt, baking soda, and mica powder. If making more than one color, divide the mixture into separate bowls.
In a small measuring cup, whisk together essential oil, coconut oil, and water. Note: If making more than one color, divide the mixture into separate bowls.
Very slowly (to avoid as much fizzing as possible) add the water solution to the dry ingredients. Stir and mash together until it resembles sand.
Immediately transfer bath bomb mixture into bath bomb molds, pressing mixture tightly into each side and adding a little to overflow each side of the mold. Press the mold together, twisting until closed.
Allow bath bombs to rest (and dry) for at least 24 hours.
To remove the bath bombs from the molds, tap the outside of the mold gently. Then, twist each side of the mold and pull to remove the bath bomb.
To give as a gift, wrap tightly in plastic wrap or place in an airtight jar.