The bright frock, which retails for around $560, had short sleeves and an A-line skirt, and she paired the look with suede pumps by Gianvito Rossi and a nude clutch.
Kate wore her hair in a elegant half-up style for the occasion.
The royal couple made the visit as part of their Heads Together campaign to raise awareness around mental health.
"As a parent, and as a mother, having that feeling that there is somebody there that is non-judgemental, that can provide the professional support, can really provide a helping hand at a really difficult time," Kate said.
The pair met wit some of the volunteers - one of whom was comedian James Cordern’s sister - Ruth who man the phones and take over 1000 calls every month.
Speaking after the visit, William told the charity's chief executive Sarah Brennan: "It is important that parents understand that you can't be brilliant at everything. It is totally fine to talk about it and to seek help and to speak out, because we're not all superheroes.”
“There is a lot of pressure on parents, and most of it is self-made by parents themselves, where you feel you have to be able to handle everything. You have to show strength and resilience to everything. But there are some times when it all gets too much and you need to reach out, and that's totally fine."