As I outline in Step Three of Making Fizzy Bath Bombs, add the liquid colorant to the water/borax mixture before you add your 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. (Remember, you can always add more water later.) Shake well...and mix the wet and dry phases together. The color will be evenly dispersed throughout the bath bomb like the green ones in the previous picture.

