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.
These are water based liquid dyes that are supplied in a concentrated form so we would recommend testing a small quantity first to achieve the colour required. If necessary they can be diluted further with deionised water.
All colours can be mixed to create other colour combinations so have a go and create some variations of your own.
It is actually quite difficult to achieve a vivid colour in a bath bomb as you are starting with a white base so don't be tempted to add too much as when the bath bomb hits the water it will turn the water a very deep colour.
We always recommend water based dyes over pigments for bath bombs.
However, we would always recommend testing a small amount with your formulation before starting any large scale production.