Cold will only help tighten the blood vessels to stop bleeding/bruising. It does little to help with an existing bruise. Try getting some Arnica. That's a staple in the BDSM community for bruises, heh.
Mayb next time you'll think twice about allowing someone to give you a hickie in a rather revealing place! In my opinion Id agree with other members tho, why allow them to happen? You shouldn't be getting nasty with other members as you posted it so need to be able to accept other members opinions. Me personally can't stand hickies!