In this Sunnybrook mold, it’s easiest to do swirling with a super white base and a bright vibrant color. One tried’n’true method is to take out 10% of your soap recipe and mix in the color super dark. In this example, we used blue iron oxide and pumice for texture.
Here I am, pouring the white fragranced base soap into the mold, having done one white/blue pour already. Notice how the blue color rises to the surface.