A powder room is the one place I tell people to be brave with color. You are only ever in it for a few minutes, so a wall that would feel like too much in a bedroom becomes a small, welcome jolt instead. That is why the lead here is a real tide pool aqua — saturated enough to make the room feel intentional, but softened with gray so it never tips into a swimming-pool brightness.
The work of keeping that aqua warm falls to the supporting colors. A warm linen carries the vanity and any open backdrop, while a soft chalk white on the trim and ceiling gives the eye a clean edge without going stark blue-white. Then toasted walnut on a wood vanity top grounds the whole scheme — aqua and brown are old friends, the way sea glass looks against driftwood.
Save the deep teal slate for the smallest surface you have: a single cabinet face, an interior, or the back of a niche. Used on roughly one-fifth of the room or less, it deepens the aqua instead of competing with it, and it gives the eye somewhere to rest.