I don't know how it was done in the ad, but this is how I would do it. I've not done this with that size material, but I think the concept would be the same.
First, find two pieces of wood the colors you want.
Then, attach them together. Some use double side tape, I like to use screws that will be cut off later. You have to make sure you have at least two screws on each side of the cut. For this demonstration, I'm using two pieces of the same wood sliced in two with my band saw. It'll work the same with two different species.
First cut.
Second cut.
Then glue everything together.
Sand it down and get this.
A couple of things to keep in mind. If you look close, you'll see there are a couple places on the curve that are not perfect. The reasons for this is twofold. First, this should be done with, at least, an 1/8" blade with lots of teeth on a bandsaw, or better yet, a scroll saw. Presently, I have neither. I used my standard 1/4" 6tpi. Second, no matter how small a blade you have, you have to cut the curve within the blade's curve capacity. When doing this type of work, you have to be very conservative with the curve you cut.
I started doing this to put hardwood tips on kayak paddles. A straight line joint was not very strong. By adding curves, I was able to add strength to the joint, and more glue surface. I've seen this process used on cutting boards also.
John