Colouring oil - chocolate look alike
If you really want to color oil, then oil-based food coloring would probably be the most straightforward thing. You should be able to get enough colors to mix to get the brown you want.

That said, I'm not sure how oil is ever going to be a chocolate look-alike. It might be the same color, but even melted chocolate is way thicker than oil; they aren't really going to look the same even if you get the color right.