LabColours (link to Voyageur Soap & Candle, but available elsewhere), which are suitable for melt & pour soaps, surfactant mixes and yes, even our lotions. (Although I don't know how I'd feel about using a purple lotion!)
As a note, Brambleberry also sells the LabColours, as well as ones that are specialized for bath bombs so they don't set off the fizz and ones that are specialized for CP soap.
bath bomb creating strange and funky colours, so try it out and let it sit for a few weeks before deciding to use that colouring in a huge batch you're giving next Christmas!
I have used icing colours from the cake decorating store in my products, and they worked well. I didn't have any weird reactions from the bath bombs, bubble bath, or body wash made with it. They were gel icing colours, and they didn't set off the fizz of the bath bombs, which was pretty awesome. And they were great for creating black in an anise scented bubble bath!
iron oxides, ultramarines, or other pigments of that nature as they can have some chemical reactions with our ingredients and create some horrible horrible smells!
We've made some lovely shimmering bubble bath using micas for some children and they loved it, so I can say this was a rousing success! And people like to add micas to lotions to offer a little shine and glimmer.
Make sure that any colourant you are using in a product is suitable for that product. Some of our colours say they aren't lip safe, so keep those out of lip balms and lipsticks.
You can get cosmetic safe glitter as well, which is a great addition to things like bath bombs or fizzing bath salts!