Q: iPhone 5C tells me it's too hot to use when it's cold

The iPhone is clearly too cold not too hot. I appears that Apple is using the same error message for too hot or too cold. What it really means is that the temperature is out of the operating range of the iPhone which is 32ºF - 95ºF.

See here for more information on this: Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support

Note that some temperatures may be too high or too low to safely store an iPhone let alone operate it. Also just because the Samsung phone didn't complain about temperature doesn't mean that it is good for the phone to operate it at low or high temperatures. One big problem with low temperatures is that condensation can form inside the cold phone when it is brought into a warm environment.

If you want to complain about the inaccurate error message use http://www.apple.com/feedback/iphone.html

Hi If you keep it in a parked car at that Temperature you will damage iPhone Leave it at home I would use an old phone for leaving in the car. Cheers Brian