The Rattlers
The Third uses a white with green highlights color scheme.
Per FM:Free Worlds League, page 38.
Other references:
Orion, FM: Free Worlds League (plate 5)
Numerals on the shoulders of the unit’s BattleMechs, placed directly below the Legion’s insignia of a Terran rattlesnake, identify the regiment. Per FM: Free Worlds League, page 38.