I don't think you can change the color of the LED light on the iPhone. In general color of LED light is not adjustable unless multiple color LEDs are used which is a bit more complex and bulkier than the LED on the iPhone.
It would be simpler for Apple to add a color filter for the photo app as they do now with a few existing filters.
The iPhone 5s and 6/6+ have dual LED's with different color temps for automatic adjustment. You can see them on the back of the phone by the camera. I've actually been able to see the illumination adjust while panning across different colored scenes since I can turn on my "Flash" to illuminate while shooting video.