Personally I prefer small pink nipples, but like everyone else has said that tends to vary from guy to guy. I've heard that tea extracts can darken them temporarily if that's what you're looking for. Maybe henna, although that might look pretty fake.
There have also been numerous advances in "permanent makeup" tattooing, which might apply in your situation (although lightening is out of the question that way) Skin bleaches may lighten them if that's the way you want to go tho.
