Is that a figured ribbon separating the two colors? If so I think it is just the look of that piece of wood. Maple is one of those woods that gets its look not just by grain, but in how its pore structure (it has pores, jut very tiny) reflect light. If you get a piece cut where the pores change direction. it has a very uneven appearance and can age out to look like 2 different coloirs (one area drinks up more finish than the other). I have stacks of maple necks, some with original finish, some raw, that have a similar look. Usually it has a sort of curly ribbon of various widths separating where the change happens. But not always.