I shot my WhiBal card and got 5850, 5900 and 6100; That was each light on well-charged AAs and the same setup. 6300 seems a little high but as long as it is consistent then I don't think it is worth much trouble.
Thanks Jim for helping out. I am guessing your 3 numbers are for power 1/1, 1/2 and 1/4 respectively.
The change in color temp is 250K from 1/1 to 1/4. Mine is 300K.
Looks a little better than mine I guess. This tells me 600ex-rt's color temp does vary considerably with power level.
Also the absolute color temperature value does have manufacturing variations
as much as a few hundred k's.
Just tried my old 580exii again just to compare:
Color temp on 580exii seems a lot tighter with respect to power.
Thanks to all who had replied!