The various OPFOR uniforms at the different CTCs are based on what mission set or capability that specific OPFOR organization is representing.
The different roles require different uniform items (and/or grooming) - and these assignments usually are static within the OPFOR units (not something picked on a whim by a company/troop commander). Some OPFOR elements are designated as uniformed, conventional forces (here is usually where you see the black/tan/od green/dcu uniforms). Other OPFOR elements are designated as 'special purpose forces' (irregular, guerilla types) - this is typically where you see guys with non-standard equipment, uncommon uniform patterns, relaxed grooming, etc.
As far as only 1 platoon/section in an HSC/HHC wearing something non-standardized, this tends to be the element that otherwise would be the Battalion Scout platoon/section. At the end of the day, its still the big Army and a similar organizational manning structure is loosely maintained across the OPFOR forces. There's still weapon squads, mortar sections, heavy weapons sections, etc found amongst their ranks, along with the same expectations for skill development throughout their career progression that is commensurate with their peers in maneuver units.