Why are my oven knobs turning pink?
Yes, as @brhans says, that definitely looks like copper has been exposed, so there is no way to go back to the nickel/aluminum color. (Copper turns that color when exposed to vinegar.) I would personally try to use more vinegar on the non-pink parts of the knob to try to get the rest of the nickel/aluminum to "reveal" the copper and get a uniform copper color, since there would be no way to "restore" the nickel/aluminum.