But Clark is going with the Authority to Warworld after he's formed them, and he doesn't form them till Superman & The Authority. Batman/Superman & The Authority special is also set right after Clark is kicked out of the JL and all of the stuff with the Warworld fragment in the current arc has happen, and he's also *just* formed the team.
My guess is that the gray hair is the cell decay from the radiation from the breach Batman and The Atom told Clark about at the start of the arc. I mean, that's what gray hair is after all-- it's just cell decay in the hair. I think Superman & The Authority takes place while Clark over the span of issues 1-3 of Son of Kal-El. In those issues Clark is still on world, but has taken a step back, and then in issue #4 it says he leaves. Over the span of that time my guess is the radiation really sets in and his powers start to go along with the color in his hair.
Dispenser of Truth had the great theory that Clark is wearing his classic suit and has the color back in his hair on Warworld because Manchester Black is projecting that image with his powers, and the rest of the team is making up for Clark's flaws in secret. PKJ said that the Authority would be using covert ops and gorilla warfare while on Warworld. Dispenser of Truth thinks this is how they'll basically fake Clark's powers for him. And the reason for this is because during that same issue when Batman and Atom were telling Clark he was weakening, Clark said that he wanted to keep this a secret because he didn't want guys like Lex or Darkseid to find out, so one would have to assume that includes Mongul.
It's a really sound theory.