The discolouration of the bladder could be down to the development of a bacterial film or mould if the bladder is stored damp at room temperature. It happened to me once when I forgot to follow my cleaning regime and left it to fester when I shouldn't have. If so a gentle scrub with a cleaning brush should shift it. The film came away easily and I followed it with a rinse with Milton. The problem I find is the Milton has a lingering after-taste that becomes apparent on the next walk, especially from the tube, so I only use it sparingly, Normally a hot water rinse, shake dry and freeze works for day to day. By the way, it you freeze it, leave the top off, if can be a .... to unscrew if you forget to defrost it and any water in the threads freezes.