I can tell you what I did...but everyone here is more experienced. I did a coffee soap with cocoa. I made my batch, divided it, put cocoa in and mixed it up real well, then swirled it into the pot with the rest of the soap. Then took the handle of a wooden spoon and just lightly dragged it through the soap batch and poured it into the mold. It looks awesome.
I use a spatula at trace, before or after pouring, and "cut" in the color like you would to make a marbled cake. You can also use a fork after pouring and stick it in straight to the bottom and straight up and then drag it across the surface for a bit. That is the way they do those nice patterned papers in old books.