Ultramarine is a deep, strong blue — not a pale sky blue and not a dark navy, but a rich blue that looks almost lit from inside, with a slight purple lean. Think of the blue in old religious paintings, on the Virgin Mary's robe. It works both as a noun (the colour or the paint itself: "a tube of ultramarine") and as an adjective ("an ultramarine sky", "ultramarine blue"). It is not an everyday word the way "blue" is. You meet it where colour has to be named exactly: painters, designers, people choosing paint or fabric, and writers reaching for something more vivid than "blue". So it feels slightly artistic or technical, and using it makes what you say sound more careful and more beautiful. The history is why the name is strange. Originally the pigment was ground from lapis lazuli, a stone that had to be brought to Europe from far away — mostly from Afghanistan — by ship. So it was "the blue from beyond the sea", and it was famously expensive, at times worth more than gold, which is why old painters saved it for the most important figure in a picture. Since the 1800s it has been made cheaply in factories, so the colour is now common and the name has kept its old glamour.

群青（色）；深藍。畫材行、顏料管上就寫「群青」；一般聊天講顏色的話，台灣人多半直接說「深藍」或「寶藍」