I gave a CD a swipe with both sides of the stone & the white side left coarser scratches.
On steel they both leave such a mirror polish its hard to tell but if you scratch plastic with it its more obvious.
I've honed a whole bunch of razors with it over the last 12 months & always used the white side as 3k. I've retired it now as I found it too soft for the amount of work I do but that was how I used it.