Princess Charlotte, four, joined her siblings and parents last week for the weekly clap for carers campaign in a special television appearance on BBC.
All the family sported blue and The Duchess was seen in a £129 Ghost floral dress.
Blue is said to have a calming effect, and represents trust, loyalty, wisdom, confidence, intelligence, faith and truth.
The makers of the dress have revealed online proceeds from the dress will go the NHS.
The dress is currently sold out but Kate has sported Ghost fashion multiple times and they have numerous designs on their website.