ceruleum

Ceruleum is a term used to describe a blue color associated with the sky. The word derives from Latin ceruleus, meaning sky-blue, and is a historical form related to the more common modern term cerulean. In English and in historical texts, ceruleum has appeared as a descriptive color in natural history accounts, art treatises, and poetic descriptions of the blue hue of the heavens or seas.

In the context of pigments and dyes, ceruleum is sometimes linked to the blue pigment known as cerulean blue. Cerulean blue is a cobalt stannate pigment widely used by painters since the 19th century; early or older sources may have used ceruleum as a generic name for this or similar light-blue pigments. Because color naming was less standardized in earlier periods, ceruleum can appear in antique recipes or catalogues as a color designation without a precise chemical specification.

In modern usage, ceruleum appears mainly in literary or historical discussions of color and is not a standardized term in contemporary color systems. When precision is needed, designers typically use cerulean, azure, or blue to convey the intended hue.

Related terms include cerulean and cerulean blue, both of which more specifically refer to shades in the blue family and to the modern pigment cobalt stannate.