Sorry just saw this, there are numerous coatings on the market for clays and concretes. Isonit, thermilate, Renotec etc all coloured coatings.
Used to be the case the manufacturers state roof has to be pressure cleaned in order to validate guarantee, Hence soft washing is no good, not sure if this is still the case.
Once cleaned you apply fungicidal wash to eliminate any remaining moss spores.
Coatings are applied via airless spray, usually two coat application first is diluted as specified and acts as primer coat.
Generally coatings have 10 -15 manufacturer guarantees