。
She was wearing a pure-white dress for the wedding.
Literal
She [topic-は] wedding [for-のために] pure-white [possessive-の] dress [object-を] was-wearing.
純白 ('pure white') stacks 純 ('pure, unmixed') with 白 ('white') for a heightened, almost ceremonial whiteness, the perfect adjective for a wedding gown. The Western-style white wedding dress only became common in Japan in the postwar era; the traditional bridal garment was 白無垢, a pure-white silk kimono carrying a similar symbolism of purity but utterly different in cut and meaning. Modern Japanese weddings often feature both — a 神前式 ('Shinto-style ceremony') in 白無垢, then a Western-style reception in a colored ドレス. The verb pair 着ている ('is wearing' — in the resulting state of having put on) is the standard way to describe attire, since 着る is technically a punctual verb.