blue-gray vs ultramarine

blue-gray

adj

Of a bluish grey color.

ultramarine

adj

Of a brilliant dark blue or slightly purplish colour like that of the pigment (noun sense 1).

noun

In full ultramarine blue: a brilliant blue pigment that was traditionally made from ground-up lapis lazuli, and now usually either extracted from mineral deposits or made synthetically.
A brilliant dark blue or slightly purplish colour like that of the pigment.

How often have the words blue-gray and ultramarine occurred in a corpus of books? (source: Google Ngram Viewer )