It's a brass knob. Mostly good for indicating other brass components in the rest of the board, and adding a bit of pop vs the usual color matched knob.
The white one looks great if you add a pop of color to your keyboard but with the all white keycaps it came with I quickly switched it out for the brass one.
The gold is a nice color - though I wish it had the surface texture of all of the other knobs. The flat look isn't totally for me though I do admit it still looks nice.