The classic blue for winter coats, blazers and skirts associated with and named after the British naval uniform.
Plain navy as such isn’t used a huge amount in furnishings but when it is is very effective, and a very good colour for teenage boys. Oranges, reds, yellow, dark grey, white of course and all of the oyster, cream and off whites work well with it.
Where we do see navy is as indigo, the deepest dyes that are almost black. Very smart.