These are specifically used in Cold Process soap making. Mix by removing some of the soap mixture into a small container, add the colorant, from 1/8 tsp up, depending on the depth of color desired, and a quick whir with the stick blender, then add this back into your main mixture to swirl or to color the entire batch.
It can be mixed with oil outside the soap pot as well, but the method above is easiest and you can judge the color you will get more accurately. These colorants are certified soap safe. The best thing about oxides is that they are a "what you see is what you" get type colorant with the exception of Purple shades.
INCI: Iron Oxide (CI 77492)