In a medium bowl, mix together all of the dry ingredients. Add as much or as little color pigment to achieve desired look. Whisk to combine.
In a separate small bowl, mix together the wet ingredients.
Add the wet ingredients to the dry ones very slowly. (This is very important; otherwise you will activate the citric acid which is used to create the fizzing affect.) Mix until combined, and the ingredients look like wet sand.
Fill each half of the mold, packing the mixture in slightly until it's overflowing. Press the halves together firmly. Let the filled mold sit for one minute. Lightly tap the mold and gently pull it apart to remove the bath bomb. Let the bath bombs dry for 24 hours before enjoying in the tub.