iPhone 6's screen is too bright???
You haven't used a Samsung @ Night with their Super AMOLED screens, give it a try and you'll see even at the darkest settings it will be beyond too bright.
I thought the Samsung phones Super AMOLED screens were really dim? Maybe they just had a small range of brightness or something, making them be too dark for outdoor viewing, but too bright for super dark rooms. I just remember my Galaxy S4 always being too dark.
But seriously guys, just go to Settings>General>Accessibilty>Zoom>On. Make sure it's set to "Full Screen Zoom". Also, go to Settings>General>Accessibility>Accessibilty Shortcut>Zoom.
Now, triple tap on the screen with 3 fingers to bring up the zoom options. First of all, make sure it is set to "Zoom Out". Then, do "Choose Filter", then, "Low Light"
Now you should be able to triple click the home button to disable zoom, and triple click the home button to enable it. Since you are set to "Zoom Out" in the zoom menu (Accessed by triple tap with 3 fingers) only the "Low Light" filter is applied. Essentially giving you very quick access to lower brightness levels. Just triple click the home button when you need the screen darker.