Ultramarine is specifically a deep and vivid blue, suggesting richness or depth. The evening gown was a striking shade of ultramarine.
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Not Just Any Blue
Ultramarine indicates a specific blue, not light or sky blue, but a more saturated one. He chose ultramarine, not aqua, for the book cover.
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Luxurious Association
Historically, ultramarine was expensive to produce, often associated with luxury or nobility. The ultramarine trimmings featured in the royal tapestry.