The collection trays should be black or as dark as possible. I've even seen a transparent mirror film applied to the inside of the glass,(Top), so that any solar rays that might be reflected back out, are bounced back onto the collection tray, or wick. This can make cleaning the collection face a bit pickier though.
The unit can get pretty warm in direct sun light this way, but it also works all that much faster. Just make sure that the reservoir to be purified is keep cooler than the actual still interior. This way the water will gain a better capillary motion through condensation and will draw faster, giving your still a higher yield of portable water.

Michael