In 1969 Coster joined the roster of principal artists at the Cologne Opera, making her debut there as Vitellia in a new production of Mozart’s La clemenza di Tito.
At that time this particular Mozart opera was rarely performed and the production, directed by Jean Pierre Ponnelle, received high acclaim from music critics internationally.
In 1975 she sang a brilliant Eboli in the Cologne Opera's production of Verdi's "Don Carlos".
Returning to Britain she was Amneris in Aida for the English National Opera and undertook a considerable amount of Broadcasting for the BBC.
Coster has participated in a number of important recordings, including Léonard in Les Huguenots by Meyerbeer with a cast including Dame Joan Sutherland, Dame Kiri te Kanawa and Martina Arroyo and Adriana Lecouvreur by Cilea with Montserrat Caballé.