To be honest, i'm not sure why you'd want to lighten the color of your nipples.. Most guys find it attractive for lighter skinned girls to have darker nipples.
In the end though, you have to do what makes you happy. And yes both skin lightening creams and bleaches will work to lighten your nipples. Bleaches are safe only as long as they are no cuts or anything of that nature involved.
There are also perminant makeup options that will work for this. May need to be reapplied in around 8-10 years. It's a surgical procedure.