Dundas is a properly assured blue — the kind of colour that does the heavy lifting in a room and doesn't want competition. The trick is to give it contrast and a bit of warmth, not to throw more colour at it.
Start with your anchor. Run a cool, crisp white across the woodwork and ceiling — Farrow & Ball::All White (LRV 92) is exactly right here. It's a clean white with no yellow undertone, so it keeps the edges of the blue sharp and gallery-like rather than muddy. That high-contrast frame is what makes a deep blue read as deliberate rather than gloomy.
Then layer in weight. Bring charcoal and ebonised wood into the mix — dark furniture, a near-black mantel, smoked oak. If you want a painted partner in the same tonal world, Paint & Paper Library::Blue Blood (LRV 16.4) sits beautifully alongside Dundas; it's dark enough to ground the scheme without fighting the main blue.
The most common mistake with a scheme like this is letting it tip cold and clinical. That's where one warmer note earns its keep. A flax-linen textile — natural undyed linen on a sofa, blind or large cushion — is the easiest way to introduce it, but if you want it on the walls or in a secondary space, Mylands::Beehive Place No.140 (LRV 58.6) gives you that soft, sunlit warmth that stops the blue feeling like a fridge.
Want a punchier accent? Dulux::Fuchsia Falls 2 (LRV 29.8) is the unexpected play — a rich pink-magenta that sings against deep blue when used in small doses, a single chair or an artwork. Use it sparingly; it's seasoning, not a second main course.
Practical advice: test Dundas on the wall that gets the least light first. Deep blues drop fast in poor light, so if it still reads as blue and not black in your gloomiest corner, you're sorted everywhere else.