As per this thread from 2004 on slashdot.org, high contrast is the way to go. Some of the commenters also note that the human eye is more sensitive to greens and yellows.
I went into paint and made a bunch of test images. The images were just a solid color background with solid text of another color in various font sizes. I took an HP Slate with the images outside and flipped through them. It was mostly cloudy yesterday but still quite a bit brighter than inside.
- Black on White was the most readable.
- White on Dark Green was probably the second most readable.
Anything with a blue background around the shade of the sky was the worst of the bunch (not really a surprise there).
As AudioDroid commented, white backgrounds will do their part to drain battery life. Pure black (#000000) backgrounds are typically not liked by a majority of consumers. However, dark colors with white or almost-white greens and yellows are almost as good and give you a little bit more color to play with.
It is also worth noting that the HP Slate 500 has a VERY reflective screen, its almost like looking at a mirror and dark backgrounds make this more apparent. A matte finish would have helped quite a bit, just something else to keep in mind.