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.

I'm not sure what op was looking for, but I often mix (low-ish grade)olive oil with balsamic vinegar for a salad dressing. That could look dark chocolate like.

Oil does not always blend or emulsify well, so this might be a challenge, it would all depend on the use for the ops chocolate looking oil.

I do agree don't think a oil based product like sunflower or olive oil is the ideal liquid to mix but rather water with soy sauce or just chocolate sauce itself if you want it a bit less runny.