My impression was that possibly some real science was done here, but the actual results from those investigations are cunningly hidden - it implies that no explanation was forthcoming, but it's possible that they did explain it. For example, I find it hard to believe that a good chemist given materials of different colours (ie with image on and with no image) could not find a difference to show why one is darker than the other. The report states that no dyes or paints were found, but does not say what was found, you follow me?
Authorities did get involved, after all, and scientists don't have to come to investigate these things off their own bat - they can be commissioned to do so.