indigo vs navy blue

indigo

noun
  • A purplish-blue colour 

  • An indigo-colored dye obtained from certain plants (the indigo plant or woad), or a similar synthetic dye. 

  • An indigo plant, such as from species in genera Indigofera, Amorpha (false indigo), Baptisia (wild indigo), and Psorothamnus and Dalea (indigo bush). 

adj
  • having a deep purplish-blue colour 

navy blue

noun
  • A dark blue color. 

adj
  • Having a navy blue color. 

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