The iPhone 6 has a dual-LED flash. What are the color temperatures of the two LEDs? Are there any commonly occurring light sources or scenes that are outside this range? For example, a sodium vapor lamp at 2700K?
There is no single fixed color temperature, the so called
true tone flashis using an alghorithm to assess the current lightning conditions and adjust the color temperature of the two LEDs accordingly.
I'm not aware of any way to switch it off or control it manually, if you believe /r/jailbreak it's likely off-limits for jailbroken iPhones as well.
As for your question where the limitations are: I couldn't find any technical specifications on the color temperature range on this, the general number tossed around is that it's capable of producing over 1000 different ones without giving specific temperature numbers.