While it's a nice gesture, I feel like this whole strategy has serious flaws:
When the weapon dyes will be implemented, all recolor transmogs will lose their value and usefulness. It feels like the effort is thrown away when you could have introduced - one-by-one - a shader for the dye compatibility with a similar investment (after implementing the base of the system).
Adding recolor versions as transmogs will saturate the available so far unique roaster with cheap copies (same entry multiple times), which could be useful for supplying an alternative to dyes, but right now it will just make a mess from the transmog menu. It's already getting buggy as you have more and more transmogs and your whole loadout is seriously delayed for any adjustments after joining a server.
As a Twitch Prime user, I feel betrayed that the loot is not unique or useful, at least to me who have much better options to use. I would be happier if the loot would give something that's missing from my arsenal, or give something pragmatic to use.
The whole action feels like a band-aid solution instead of addressing the actual problem.
I know that rebuilding all weapon models with the proper texture maps and shaders for a dye system is a very huge task. I can see however a very interesting - agile - opportunity with this direction:
You could implement a simple UI tweak to group the same models together and give us the option to choose the variant on the fly. That way each new color variant is just a "dye" without actually the need for rebuilding the whole system. For now, behind the scenes, they would be different transmogs, but we would not clutter the UI with the same entries, and later the UI can be a frame for having a different type of weapon variant/level.
Later these variants could mean different dye maps or levels, so their value would not be lost. With a system like that these recolors would not feel underwhelming or as a band-aid, it could even generate excitement to collect these as part of the beginning of the dye system on weapons.