Ultramarine blue is a versatile pigment with an extensive history. In modern times it is made by taking pure kaolin clay, sodium sulfate, sodium carbonate and sulfur and roasting it in a kiln. The pigment is lightfast, and stable to air and alkaline environments. It is useful in coloring mediums such as linseed oil, acrylic emulsion, milk paint, plaster, cement. and soap.
It is very intense blue and only a little is needed for coloring. I used it quite a bit in soap making and I frequent start with ultramarine blue when mixing up paint for my art