The sharp colors and crisp contrast of your iPhone’s screen certainly increases readability although it can be rough on the eyes in a low light environment even if you reduce the brightness levels to a minimum.
There are screen dimming apps that can solve the problem of course, but as it seems you can achieve the same result without a third-party app, simply by making use of your phone’s settings.
To dim your iPhone’s screen below the lowest level available in the Control Center you’ll have to open the Settings app and navigate to General->Accesibility->Zoom. Toggle on the Zoom option.
On the same screen scroll down a bit and set Zoom Region to “Full Screen Zoom” and Zoom Filter to “Low Light”.
For as long as you keep zoom enabled you can zoom in and out by double tapping with three fingers and the low light filter will still be applied. Even if you zoom out using the three-finger double tap the screen will still be dimmed.
Alternatively you can benefit from creating a shortcut so you can easily toggle zoom on and off without having to navigate through the Settings. To do this go the Accessibility in Settings, select the Accessibility Shortcut and tap on Zoom. Then you’ll be able to toggle Zoom on and off (along with its low light setting) by clicking three times on the home button.