I moved this topic to the beginner's forum because there are lots of great opportunities for new guys to learn from the discussion, and Q&A here.
The voids are common in cable, even many long time welders will abandon it out of frustration in closing up all the voids. Much of the cable out there is all the same alloy so the only contrast possible is from the weld zones. As Butch pointed out allowing to rust will add contrast but will also increase those nasty voids and cold shuts. The nice silver and black contrast you have seen on some guys cable is from overheating it. Get it hot enough in the right/wrong atmosphere and burn the carbon out of the outer layer of the strands and then finish your welding quickly.
What I find particularly interesting about your bar is that you can make out the twist pattern, not in a distortion of the pattern but in darker bands. Perhaps another artifact of the forging atmosphere.
Everybody has the privilege of settling on their own level of ignorance. Nobody has the right to impose it on others.