Later on their true nature is better revealed.
The brightly iridescent nacreous clouds (Type II Polar Stratospheric Clouds) are embedded by less bright and colourful Type I PSCs.
Type II (nacreous) clouds are made of tiny ice crystals at temperatures of -85C and below. Type I clouds can form at slightly higher temperature (-78C) and are nastier stuff. They are solid and liquid specks of nitric acid hydrates and nitric/sulfuric acid solutions. Their surfaces are catalysts for conversion of chlorine species to more active free radicals that can then destroy stratospheric ozone.