Ultramarine blue is a classic true blue color in soaps. It is very concentrated, therefore starting at the lower end of the suggested use amount and adding more is recommended. If too much is used it can stain washcloths or make your bathtub blue!
For melt and pour soaps, add ultramarine blue to glycerin before adding to your soap. For cold process, mix in oils before adding to your soap. This will help disperse the product and help to avoid speckling.
Recommended use rate in cold and hot process soap is .5 tsp. per pound of oils. For melt and pour, the recommended use rate is .25 tsp. per pound of base.