After reviewing your post, it sounds like the screen on your iPhone 6 is very bright in certain circumstances. This can happen if a setting like auto-brightness is enabled. I would recommend you read this article and use it to help troubleshoot the issue.
About brightness levels
iOS devices use an ambient light sensor to adjust brightness levels based on the light conditions around you. The sensor lowers brightness in dark locations and raises brightness in light locations. The auto-brightness feature is on by default.
When auto-brightness is on, you'll notice that your device's brightness slider moves according to changing light conditions.
In iOS 11, you can turn auto-brightness on or off in Settings > General > Accessibility > Display Accommodations. In iOS 10 and earlier, go to Settings > Display & Brightness.
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