From Halopedia, the Halo wiki
This special project is a subset of the SPARTAN-II Program under the UNSC Navy's Special Warfare Command. Marines call these unique supersoldiers "Hellspartans" due to their gung-ho attitudes toward combat.
Five SPARTAN-IIs from the active ranks of Naval Special Warfare were handpicked for this program. Due to the possible extra strain on the body from the new biological augmentations they received, only two of the five recruits made it past the second month of in-suit training with their upgraded MJOLNIR Mark VI/EVA suits.
Unlike the standard gunmetal green paint used for most SPARTAN-II MJOLNIR suits, the "Hellspartan" EVA suit is painted in "armor livery dazzle camo". These suits are painted with dark charcoal black as the primary color, with small bits of red and white placed around the suit. The helmet has a vertical brace extending from the back of the head to the chin; the faceplate sports a pair of lime-green, eye-like patters. A small, stylized skull is mounted on the groin plate.
Numerous upgrades were added to the armor, such as an enhanced visual interface with a full-spectrum range of vision (including infrared, low/no light amplification, heavy light/flash suppression), advanced targeting matrix (including sight-linking and multi-target tracking systems), as well as custom armor plating.
The "Hellspartans" are a nod to Spawn, the most successful creation of Todd McFarlane, the founder of McFarlane Toys. The Hellspartan action figure was limited to 3,000 pieces and was only available at the 2008 San Diego Comic Con.