That is a thing I can't answer Steve.
We never had flux coatings when I was 'into' brazing and bronze welding. We used to use a system the same as silver solder. Seperate flux and filler rod.
If they come pre packed, there should be some info on the packaging as to the use of the different colours.
I would think it is temperature range rather than material designation. But on the other hand, with silver solder you can purchase the flux for material types. I now use Tenacity 5, which is designed for stainless steel, but works well on everything else.
I do have a small quantity of very high temp silver solder, that was used for soldering up the pipes on the wagons that have banks of high pressure cylinders on the back. I have never been able to melt it, even with a large propane burner. So very high temp silver solder must be available, and if you coupled that with Tenacity 5 flux, then I think you would be onto a winner.
Bogs