This is a follow-up to the solar water purification project.
I found back plastic cups at a discount party supply store. They're a bit smaller than the blue ones, 12 oz rather than 16 oz, but I tried a head-to-head comparison. It turns out that a black cup with a white liner seems to roughly double efficiency over a blue cup with white liner, though it's still too low to be useful.
Weather conditions: broken overcast at 11:17 when the test started, becoming clear by 13:15. The stills became shaded at around 18:00.
It was difficult to accurately measure the fairly small amount of water they produced, but the blue cup produced just over a teaspoon of water, while the black one produced about two teaspoons. One of the difficulties is that I measured only the water that accumulated in the bottom: there were still a fair number of water droplets around the inside of the bottle.
The next things to test will be adding a reflector and raising the cup nearer the top of the bottle.