Different rusting solutions on the same piece of steel will give different shades of color. Different steels with the same soln,,again different shades.
Change the color further as Dave says by altering the polish, rust time, carding procedure and you can see there are lots of variables. Even the water used to boil can effect the color. Clean soft water the best to change over the oxide.
Generally, the coarser the coating and rougher the polish,,the darker (blacker) the color. Probably because it doesn't reflect as much light back at you but that's just my guess. I'm no scientist!