The mercenary's guild's biggest resource is power. Manpower, specifically-- its members are often coursers who provide their literal strength to others. Its circles often focus on providing the same service.
Hired individuals include mercenaries, assassins, guards, aegis, and any other skilled fighter. Heroes, that is, adventurers who assist others on any menial tasks, also join the Mercenary's guild, working on low pay to lend strength to those who cannot help themselves, using the network to find those in need rather than those with wealth. The Mercenary's guild is a two-sided coin of the most and least noble of individuals, and there is a lot of inner competition.
These are examples of appropriate colors for a circle of the Mercenary's guild.