For the non-EVE players, when you join a fleet in-game, the other members of the fleet have a purple icon next to their name. NPSI is a play-style where you join transient fleets with the express intention of getting into battles.

Players in your own corporation and alliance typically have blue icons, and those you're at war with have red icons- this is based on manually-set "standings". Alliance roams typically have a NBSI policy: Not Blue, Shoot It!, which means you'll be attacking enemies and neutrals.