CertainTeed covered this under warranty as we had mold on the roof, the company they contacted did an excellent job, around 2 years ago. I think I still have the paperwork. In any case do not ever use a high pressure system. I would call the manufacturer first and see what their thoughts are. Over the years the old shingles lost some of the stones and this will probably result in some color change, and this is probably not reversible with washing.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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