I'm sure there are many ways, but here's how I would do it. First turn the blank round. Then use an index plate on the lathe to hold and rotate the blank as necessary. I would build a small platform with a dowel or rod that sits in the tool rest hole. With a small bit in a dremel tool, I would use the platform as a guide to keep the tool in the center plane and run it down the blank creating a channel. You can then fill the channel(s) with colored resin or epoxy or veneer slices.