The explanation is lot simpler in practise than most comments have explained so far. From my own experience, in lame terms, if you use non-black caps, for argu

The explanation is lot simpler in practise than most comments have explained so far. 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.jpg http://www.kveller.com/images/Article_images/film-canister.j... will prevent the non-black cap from stealing sun rays from the water.