Prussian blue

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An insoluble dark blue pigment and dye, ferric ferrocyanide or one of its modifications.
2. n. See iron blue.
3. n. A moderate to strong blue or deep greenish blue.

Wiktionary

1. n. inorganic chemistry An insoluble dark, bright blue pigment, ferric ferrocyanide (equivalent to ferrous ferricyanide)
2. n. A moderate to rich blue colour, tinted with deep greenish blue.
3. adj. Of a rich blue colour, tinted with green.

GNU Webster's 1913

1. n. See under Prussian.
2. adj. (Chem.) any one of several complex double cyanides of ferrous and ferric iron; specifically, a dark blue amorphous substance having a coppery luster, obtained by adding a solution of potassium ferrocyanide (yellow prussiate of potash) to a ferric salt. It is used in dyeing, in ink, etc. Called also Williamson's blue, insoluble Prussian blue, Berlin blue, etc.

WordNet 3.0

1. n. any of various blue pigments
2. n. a dark greenish-blue color