I suspect the chemicals were shipped from the manufacturer in light reducing containers because the containers would likely sit on an open shelf at the dealers'. I think that keeping the chemicals in a cabinet would protect them from light regardless of the type of container.

From my personal experience, not a scientific test, I have come to believe that a glass container keeps chemicals longer than a plastic one, even if not full. I've read that many kinds of plastic will breathe oxygen, and since the outside surface of the container is relatively large, it allows chemicals to oxidize more rapidly.
juan