I only use natural colour in my soaps. I have read a couple of articles that suggest that mica powders, biodegradable glitters and other general soap dyes are actually having a negative effect on the environment. Whilst I haven't found anything definitive for either side of the arguement I decided I'd prefer to stick with the stuff that I have more confidence in - herbs, roots, clays and spices.
Just a few of the colourants I enjoy using:
Alkanet Root - infused in oil before making the soap, this root gives the oil a gorgeous deep red colour which turns the most beautiful deep, luxurious purple when it hits the alkaline lye solution. This then becomes a beautiful lavender colour in the cured soap. I love using this as a colourant, I love the way the colour changes throughout the process!
Turmeric - This one can go anything from brown to orange. I like to infuse this one so I get a smooth soap and it results in a lovely orangey brown.
Clays - Clays come in various colour and can also add a bit of slip to the lather. I use pink & green clays in the Fresh Garden soap.
There's lots of other things out there that can colour soap naturally and there's other ways to use the colours to make the most out of them too. I love to experiment :)