The best type of Colours to use in your Bath Products are the liquid dyes. Pigments could possibly stain the bath if you use them in Bath Salts or Bath Bombs.
We would recommend testing the colour on a small sample first, especially with salts as they may not always give the colour you expect.
Remember. when using the dyes in Bath Bombs, it will look very pale. Do not be tempted to add more because as soon as the Bath Bomb hits the water the colour will become much deeper. It is hard to colour a white base (as in the sodium/citric base) a deep colour.