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I am going to paint my Tau in the color of the Khorne ( Dark red with Gold ) meaning they are most likely to be in the same colors that the Farsight Enclave have.

The painting scheme is going to be not shiny new Tau suits that you often see, but battle fatigued suits where the colors are kind falling off or burned off. ( Basically first spray em with black, paint whats needed in Red and Gold and then dry-brush over in a bit of silver and wash over the silver to add a dark edge to it to simulate burns )

But the deal is that the enclave is basically "unknown" what they are really up to, might they be the unspoken heroes of the Tau against the chaos? or are they the only Tau that ever fallen to chaos?
Did the lack of Ethereal presence finally free Farsight of his superior and open his mind to greed of power to go do his own thing? Or maybe better simulated as a "pre-fallen" Farsight army?

So a suggestion to what kind of deal these Tau have in a fluff?

I think it's deliberately open ended so you can do whatever you want and have it "fit" with the canon.

Edit: So yeah, all of your suggestions would work.