Ummmm....gas.
All kidding aside, it's a grey-ish gas that hangs out at the bottom of the keg. Maybe it has something to do with the temp difference, the StarSan or something else but after hooking up an empty keg, blasting it with CO2, purging the air and then removing the lid it hangs out down at the bottom.
Or maybe I'm high....
Weird...I've never seen that. Maybe it has something to do with temperature, as PVH suggested. Is your tank in the fridge? In general CO2 is colorless.
In any case, your post inspired me to do some home science. I took a corny and ran it through four pressure/purge cycles to make sure the contents were as close to pure CO2 as I could get. Right after opening it, I stuck a lighter in and it immediately went out. Then I sealed it up again, did the pressure/purge thing again, opened it. After twenty minutes, the lighter still went out, albeit more slowly. After an hour, it stayed lit.
In open fermentation, I've got to imagine the CO2 would blow off as fast as it could be made. Even in the couple inches of headspace provided by a standard ale pail, my hunch is that the CO2 would get disrupted by whatever turbulence is created while futzing with the beer. The CO2 blanket doesn't stick around for long in an open environment. Brownian motion FTW!