。
She wore white to the party.
Literal
She [topic-は] party [at-で] white clothes [object-を] was-wearing.
白い服 ('white clothes') uses the i-adjective 白い ('white') as a direct modifier; the colour 白 also exists as a bare noun in compounds (白ワイン 'white wine,' 白雪 'white snow'), but as an adjective for clothes the い-form is required. 着ていた (the past progressive of 着る) describes wearing as an extended state across the party rather than the act of putting on. In a Japanese context, white at parties carries no particular taboo — but it does carry a strong association with weddings, so a guest wearing pure white at a wedding would be seen as overstepping the bride.