Every once in a while—for a special occasion, holiday, or season—you might want to add some color to the bath bombs to make them a bit more festive. After adding color, you can use shaped molds to make bath bombs for holidays, such as tinted heart-shaped bath bombs for Valentine's Day.
If you wonder whether the colorant you put in the bath bombs will color the bath water, the answer is that it will sometimes. However, it depends on the type and amount of colorant you use. There are a couple of ways to add color: one in the wet phase and one in the dry phase. In addition to the other ingredients for the bath bombs, you'll need water-based or water-soluble dry colorants.
Option One: The Liquid Phase
The simplest way to add colorant to a fizzy bath bomb is to put it in during the liquid phase of making the bomb. You can use either water-based or dry colorants, as long as they are water-soluble. Micas and oxides disperse in water, but they are not water-soluble, and the results in the bathtub when you add the bomb to the water can be a bit unpredictable.
As you mix the wet phase of the bath bomb recipe, add the liquid colorant to the water after you add the borax but before you add fragrance oil. If you're using a water-soluble dry colorant, add enough water to it to make it equal a half-teaspoon or so. (You can always add more water later.) Shake well and then mix the wet and dry phases together. The color will be evenly dispersed throughout the bath bomb.
Option Two: The Dry Phase
You can also add the colorant to the dry phase — or at least the first part of the dry phase. For a speckled, textured look, add the colorant to one of the salts before you mix it in with the rest of the ingredients.
For the best effect, add the liquid colorant to the Epsom salt. The grains of the salt are large and hold the color well. Put the Epsom salt by itself in a small bowl and add the few drops of colorant. Stir well with a whisk. Some of the will salt dissolve a little bit, thanks to the liquid of the colorant, but most of it will retain its shape as it takes on the color.
Mix the colored salt in with the rest of the dry ingredients and mix the bath bombs as you normally would. Some of the colorants will bleed into the rest of the mixture, but much of it will stay in the Epsom salt.
Because of the whiteness of the Epsom salt, you're never going to get a dark color, only a light pastel. If you're concerned about the colorant and usage level, test it in small batches until you're pleased with the results.