Welcome to the Bunnings Workshop community @JenCharlesworth. It's wonderful that you've joined us, and many thanks for your question on tile paint colour choices.
In the Dulux renovation range, I've been able to find a colour called Diplomatic. I'll include an image below. This is the darkest navy blue which is available. I suspect, even though it looks lighter than your image, it is fairly close. The paint in the image is mainly in shadow, making it appear much darker than what it would on a colour card. In the central area, you can see more of its true colour, which appears much lighter. I suggest your best option would be to get a sample pot of this Diplomatic colour and see how it looks in your home. Paint it on a board and place it in front of the bath. Don't paint it on the bath tiles as it is inferior to tile paint.