The Red Sea narrow range FW pH test kit should be Bromo Blue.
I doubt they refill the tank with an N2 head.

The food grade CO2 should be virtually pure CO2.
The Nitrogen head has other reasons for being used but I'd think with inert gases, they should likely not or in special cases.

I could be wrong but I see where and how they refill my tanks here, there's no Nitrogen head, it's straight CO2 and very high purity.

In other words, I'd bark up another tree, N2 gas is very insoluble anyway.
Co2 is very soluble.

Regards,
Tom Barr