This is my current favorite background and foreground color combo. I have it set as my night mode, but I use it most all of the time. It’s a nice low contrast combination, very easy on the eyes.
I tried to show this at 1% light setting, but iOS screen capture doesn’t pay attention to the light setting.
With FBReader as my app of choice, I can slide my finger up and down the screen to adjust the lighting such that it’s always comfortable. 1% in total darkness, very readable without disturbing my wife sleeping next to me.
Outside I’d probably go with black on white....but I don’t read outside that much.