So it is like Hummingbird feathers which are dull except when they reflect light. What is needed to get these Pogonognathellus down to species level?

to IDy USA Tomocerinae species from habitus shots.
There is a key in
Christiansen, K. et Bellinger, P. 1998. The Collembola of North America North of the Rio Grande, A taxonomic analysis., Grinnell College, Iowa, p.1-1520.
on Tomocerus sensu lato species.
This key uses microscopic characters to diagnose specimens. Such as : number of ocelli in the eyepatch, number of teeth in the mucro, dental spines, shape of tenent macrosetae of the footcomplex.
So it will not be possible to use this key on Bugguide habitus images...
But based on Bugguide images we were looking for new diagnostic characters based on pattern matching techniques (such as used in 'deep learning' of artificial intelligence applications). But we are not yet at that point. Currently we are in the data collecting phase. Basically we need more images ;-)

So if I understand this right, the iridescence is due to wave interference and the black occurs when the waves are in anti-phase, cancel each other out.

There appears to be two definite Pogonognathellus here: one that always reflects a shade of silver and one that always reflects multi-colors. The silver one is very common. I see at least a few every outing, sometimes maybe up to 30 or more but they don't tend to congregate like some Poduromorpha. I can go a couple weeks without seeing the multi-colored one, and then it is usually just one, maybe two. It is a little bit larger on average. If you want more photos of different individuals, I have them.

at least 8 species of Pogonognathellus in the USA. So 2 species could be present at your location, indeed.
Wrt to the wave interference. You are correct. The inphase or antiphase condition of the waves is determined by 1. the thickness of the scales and 2. by the number of overlaying scales. Also take into account that diffraction of waves happens due to which reflected colours are produced that were not originally visible in the light source or environment.
Feel free to share the images of other specimens you have available.