Is it the rind -- the darker outer shell on the bar in your photo -- that you want to replicate? If so, I don't know of a way to replicate this effect and have it be a permanent pattern. If your soap is like the others I've seen, the entire soap will eventually turn the same darker color as the rind.
A rind can happen with cold process soap as well as hot process soap and rebatched soap, so I'm pretty sure it's not related to the method by which the soap is made. I think it's more connected to chemical reactions that happen when a colorant in the soap is exposed to air/oxygen.
I have wondered if this rind effect is more common with colorants made directly from natural sources. For example, I colored one batch of CP soap with cocoa powder and the outer rind was semisweet chocolate brown while the center was (temporarily) milk chocolate. Maybe do a search with "rind" as a keyword and see what colorants come up in the discussions?
Anyways, enjoy it while you've got it!