How to get a swirl inside of handmade soap?

How do I make a swirl inside the middle of the soap like this one from etsy:
http://www.etsy.com/listing/49324931/summer-sale-wildewood-mint-and-nettle?ref=cat2_list_2

I don't want the top swirls I just want one big swirl inside of the soap.
Do I just pour two different colors into each other?
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