The problem is specifically the ultraviolet photons in the sunlight, which photoactivate the reaction between bromine ions and ozone to form bromate [1].

The visible color shouldn't matter as much as strong absorbance or reflectance in the UV band. I would guess that they're using a plastic that absorbs UV, or contains a dye that does so.

1: http://www.ncbi.nlm.nih.gov/pubmed/18290477