In a small bowl, whisk together the yellow and blue mica powders. Make sure you like the shade of green that results. Remember that the white materials of the rest of the bath bomb mixture will make these a pastel version of what you are seeing. If you would like it to be more intense, add more colorant.
Whisk together the baking soda, citric acid, cornstarch, and Epsom salts. Add the colorant combination and stir together well.
In a second bowl, combine the melted coconut oil, essential oil blend, and witch hazel together, mixing well.
Add liquid ingredients 1 tsp at a time to the dry ingredients, mixing it together slowly so as not to start the fizzy reaction.
Dust the bottom of one side of the bath bomb mold with cornstarch. The sprinkle side tends to come out on its own, so do not bother to dust that side with cornstarch.
Place about 1/2 tbsp of sprinkles in the non-dusted side of the bath bomb mold.
Immediately pack the bath bomb mold with the sprinkles full of the bath bomb mixture, completely full.
In the other cornstarch-dusted side of the bath bomb mold, pack it with the bath bomb mixture completely full with a little bit of extra, then press the two sides together, grinding and pressing them to force the two together.
Allow the bath bombs to rest in the mold for up to 24 hours before gently tapping the side of the mold to release the bombs.
Use your bath bombs immediately or store them in an airtight container until you’re ready to use them.