I also hate yellow screens and can't stand them.
I bought a launch day 5S. I instantly recognized it as a warm screen. I tried to crank the brightness and convince myself that it was fine, I'd get used to it, and that it's just the way it is.
I went to the apple store the next day. I tried not to look at the display phones. Alas, I gave in and looked. Sure enough, cool clear perfect screens of every one. I'm done. I just want the phone advertised. I returned my 5S and went back to my nice clear screened 5. I went through ~8 5's to get one I could stand. Looks like it'll be another year.
Bottom line: no it's not fixed. It's still a lottery. Good luck. Also, the 5 and 5S are nearly indistinguishable in performance.