Bourbon is just Walnut Calfskin Leather that they polish on the Black Burnishing Wheel. As opposed to being a specific dyed color like Walnut, Merlot, Brown or Black.
I actually think it's cool how they get creative to make new colors, but I understand where you are coming from here. Cause the effect probably fades with time and is harder to maintain.
Gotcha. I'd imagine it's not that easy for the average joe to turn a pair of walnut shoes into bourbons without the burnishing wheel.