Cerulean means deep sky blue. A cerulean sky, cerulean blue, painted cerulean.
It is a painter's word: cerulean blue is a specific pigment, a cobalt compound made from the 1860s, and it is prized because it stays true and does not fade. Outside painting it is used for the colour of a clear sky. Note that the Latin caeruleus comes from caelum, the sky.
1adjectiveceruleanDeep sky blue.
“We swam in a bay of clear cerulean water under a cloudless sky.”
synonyms: azure, sky-blue
cerulean is a colour.
Latin caeruleus means sky blue, from caelum, the sky or heaven. Celestial comes from the same sky.
From caeruleus + -an, from caelum + -uleus.
Etymology from Wiktionary, CC BY-SA