If it's porcelain, it's not the putty. It could have rubbed some metal off. Some sort of cleaner, maybe comet if it's a glaze.
Or you might try something softer like the liquid bowl cleaners.
I am installing a new drain on a new white (porcelain or composite, I am not sure) sink. I installed the drain with plumbers putty, but it leaks. So I took it out and repositioned it with new plumbers putty, but now there is a stain, I guess a metal stain, just off the rim of the drain on the sink. Is this a metal stain or from the plumbers putty? And how can I remove it?