For a more speckled, textured look, add the colorant to one of the salts before you mix it in with the rest of the ingredients.
I like to add the liquid colorant to the epsom salt. The grains of the salt are larger and hold the color well. Put the epsom salt only in a bowl and add the few drops of colorant. Stir well. Some of the salt will dissolve a little bit, but most of it will retain its shape and take 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 colorant will bleed into the rest of the mixture, but much of it will stay in the epsom salt, as in the red one in the next picture.

