How the heck does a flash know what color temp it is firing off?

NK Guy in his essential reference, "Mastering Canon EOS Flash Photography", says that the CT of a flash tube can vary depending on precise voltage and pulse duration. Those flashes that report CT data back to the camera monitor power levels to determine the data, providing WB is Auto or Flash.

Note that it is not only standard colour temp, but also "tint" variations (Blue/Amber and Green/Magenta) that are assessed. Degrees K is just not enough!

dmward wrote in post #16507132
But its color balance setting in the EXIF data is either a specific setting or AWB which it reads by assuming the sum of the color temps in the scene equal middle gray and makes an adjustment to get there. And is someone is saving JPGs in the camera it uses the info to process the JPG.

is it just whatever WB is set on the camera being written to the EXIF ? i know i shift WB around a lot with my cameras, and the WB is reflected in the Exif, even if the flash is fitted

dmward wrote in post #16507132
Incidentally, the gel reading feature in the 600EX only works with the three gels supplied with the flash. If one uses other gels, even stronger or weaker CTO. The flash indicates an error.

I only got two, i feel Ripped off ! (one CTO and one 1/4CTO)
both of the supplied CTO are way too red though, and horrible to shoot with, i cut my own gels from the sheets supplied with my lightstands (from MPEX, the strobist kits)

i actually have a hard plastic template that i made that makes cutting gels for the 600 really easy, i have two of the supplied wallets full of them, one Colour Correction (CTO, CTB, +green etc) the other Colour effect (deep blues, greens, pinks, reds, oranges, purples etc)

Echo63 wrote in post #16507546
both of the supplied CTO are way too red though, and horrible to shoot with

I find the same with CTO, but not with CTS (straw). I suspect that the tungsten ambient lights we (and Australia) use run at a hotter temperature compared to house lights in USA, etc. 230v vs 110v.

The camera writes the EXIF data based on its settings.
I thought there were three gels maybe just two. Its been so long I don't remember.
I found a source in UK that offers lots of gel colors all cut for the filter holder on the 600EX-RT.
Whenever I put one of them into the holder the flash icon in on the screen turns to an ! or similar which, when I dug into the manual indicated that an unknown gel is installed.