I think this discussion is suffering from some terminological confusion. My understanding, for what it's worth is this:
"HiViz" generally refers to garments that combine both 'DayGlo'TM (Fluorescent) colours, with strips or panels of 'retro-reflectives'.
The dayglo, as pointed out, works by converting light beyond the visible spectrum into visible wavelengths, thus making the fabric appear brighter than it would naturally be. Logically, this only works in daylight.
For this very reason, HiViz garments also use retro-reflective panels. These consist of a mass of tiny bead-like mirrors or mirror facets. In daylight they appear dull grey or silver, but at night they reflect back any light shone at them very efficiently. If there is however, no light source to shine at them, they don't show up.
In the absence of either technology, the best garment will be the one with the highest 'albedo' - ability to reflect light. This usually means white because white reflects all colours and absorbs none.
Unless you find yourself silhouetted against a wet, moonlit road, in which case 'ninja' black might provide better contrast . . . :-(
In general conversation I notice people tend to use terms like 'HiViz', 'Dayglo' and 'fluorescent' very loosely and inaccurately and are often vague about the practical differences. Hope this sheds some light . . . sorry.
