I believe that the "images" start as a set of interference patterns though and not really a classic photographic image. So I would still give the same answer with the exception of the holographic images of Gentet and others who may use his method which generate patterns which are turned into images.
Well, that's what's happening with any hologram. There's nothing magic about it. In the case of recordings with three (RGB) wavelengths there are actually three different holograms recorded within the same layer.
I am surprised that full colors can be created though by such monochromatic light. Or is it truly monochromatic? It may be broad spectrum but coherent. There are distinctions here! As it turns out, to answer my own question, lasers have very pure emissions of only a few nanometers in many cases. It then becomes a question of how only 3 can produce all colors of the spectrum.
The bandwidth of these lasers is quite narrow: 0,0001nm or smaller. In contrast the bandwidth of the (RGB) holograms (reflection holograms) may be around 30nm each.
Well, it seems that Gentet produces an image from the hologram which can be shifted in tone by using a special "swelling agent". He therefore appears to be reproducing the color by shifting parts of the image color. He apparently only has 4 colors in his laser line and uses this manual method during final processing to shift the color of the final image.
No, this has nothing to do with Gentet's full color holograms. The swelling thing applies to something different: so called pseudo-color holograms. It means that one can achieve practically any color with one single laser wavelength only. E.g. the emulsion is pre-swelled (the chemical most often used is triethanolamine) prior to the exposure. After exposure/processing that swelling agent has been washed out. So at the end of the process the recording layer becomes less thick than during recording. With a reflection hologram that has the consequence that the interference fringes become compressed, leading to a shorter wavelength upon reconstruction. In other words, you may record a hologram with a red laser and end up with yellow, green, blue or violet (even UV) holograms.
This is just a guess from the sketchy outline given of his process. We really don't know enough yet to make an assessment of what he is truly doing.
The real secret is in his emulsion. The very best type of his materials have an extremely small (<5nm) grain size at an extraordinarily high speed. Speculations about why that is are running high - some suspected an impact from Jacqueline Belloni (the one who introduced formate doping).
Such a fine grain emulsion provides very high index modulation and practically no noise/scatter.