Mildew will produce a green discolouration on surfaces that are often exposed to water and liquids. Something unavoidable in Britain.
Mildew is commonly seen on exteriors including roofs and shed sidings. It is a natural, and common, occurrence, but it is unsightly and can often rub off onto your clothing if you happen to brush past a surface covered in mildew.
In saying this, mildew can easily be removed from steel sheds, siding and roofs.