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 really recommend you test the colour on a small sample first, especially with salts as they may not always give the colour you expect.
Remember too, that when you are using the dyes in a Bath Bomb, it will look very pale. Do not be tempted to add more, because as soon as the Bath Bomb hits the bath water the colour will become much deeper. In fact it is very hard to get a vivid colour in a bath bomb.