Why would it make you phone even more hot? Because dark color doesn't reflect light? Somehow I doubt that the background color of the screen would have that much impact on how much sunlight is absorbed by the phone in typical outside usage (heck, if you leave the phone screen off the screen is "dark" by nature when it off, so that would seem like more of a "hazard" in that case too, yet, it definitely hasn't been).Too much black on the screen isn't practical on a sunny day either, that would make your iPhone getting even more hot (which could drain the battery faster) and make the display even harder to read.
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Clearly this is the kind of stuff that should be more important than lets say actual usability improvements of using the phone in darker environments.iOS 7 has enough problems with mistaken comparisons to Android. If it were primarily black this would just push it over the edge.