"The lamps work best with a black cap..." - why would that be so? Wouldn't a reflective cap work well too?
So a bottle that's composed of a clear plastic that naturally resists ultraviolet degradation (there are several) should be capped by a more rigid plastic (to maintain its shape). So the cap needs to be black (or at least a dark color).
A reflective cap would work just as well at first, but plastics are made reflective by being painted, and the paint soon falls off, because of ... wait for it ... ultraviolet exposure.
That's true, but it's also true that all objects eventually reach thermal equilibrium with their surroundings. Were this not the case, it would violate the law of energy conservation, and that's a law that's never broken.
In that sense, most objects, solids and plasmas in particular, are reasonable black bodies once they achieve equilibrium. Some of them show spectral lines -- emission, absorption or both -- but in most cases those lines appear on top of a blackbody curve. The sun, for example -- perhaps surprisingly, it exhibits a classic blackbody spectrum appropriate to its surface temperature, with many spectral lines contributed primarily by its comparatively low-pressure atmosphere (http://commons.wikimedia.org/wiki/File:Solar_Spectrum.png).
The state of matter least like a black body is a low-pressure molecular cloud in space, which often shows only emission and absorption lines, no classic blackbody curve. The lowest pressure gases show the narrowest spectral lines, which is how we can estimate the pressure of a remote cloud of gas illuminated by distant energy sources.
Black plastic lasts longer because 1) the UV is absorbed by the pigment, not the polymer and 2) the pigment limits UV penetration to the surface.
Here is a good doc on UV weathering of plastics and materials (such as carbon black) to absorb UV. http://www.cabot-corp.com/wcm/download/en-us/sb/UV%20WEATHER...
The point about a black piece of plastic is that the energy is absorbed at the surface -- it can't penetrate the plastic to any significant depth. That means the majority of the plastic is protected from the ultraviolet wavelengths that would destroy it.
From my own experience, in lame terms, if you use non-black caps, for arguments sake let's say you use a red cap from a coca-cola bottle, that red cap will reflect some light as well as the water. And inside the room the bottle will not "shine" as bright as it could and depending on the Sun's position the room will have red'ish rays - like a rainbow.
So using any colour cap and covering the bottle cap with old photographic film containers like this one http://www.kveller.com/images/Article_images/film-canister.j... will prevent the non-black cap from stealing sun rays from the water.