Well I'm a bit confused because the title is talking about the Persil advert, and then you asked about the Muller advert. Anyway, the Muller song is Nina Simone - I Got Life.
Is the Persil song the one which goes 'Be young, be foolish, but be happy?' If that's the one you're after, then it's by The Tams - 'Be young, be foolish, be happy'