Indigo is a dark blue bordering on purple, essentially navy blue.  It is one of the most spiritual colors of the rainbow and has been in the news the last several years, primarily, with regard to those called Indigo Children.  Well known psychic Edgar Cayce wrote much abut the power of colors and predicted the coming of the indigo children in the latter part of the twentieth century.
    Indigo is traditionally related to the energy center (chakra) in the center of the head, although in the system we follow in the International Community of Christ, it's the throat center that is indigo.  People who are strong in this color and this energy center will tend to be intelligent and spiritual.  Indigo is said to open the third eye. On the negative side, they have a tendency to lack patience and be intolerant of those who are less spiritual or less intelligent so they need to be taught to counter those tendencies and to use warm colors (red, orange, yellow) to keep them grounded.  
    Indigo gems and stones can help you gain in spiritual realization and self-mastery.  Indigo is also a color of communication but, while blue promotes outward communication with others, indigo promotes inward communication with our inner self.  Indigo can help with inspiration, i.e. ideas that "come out of the blue".  In religious art, saints and prophets are often painted in indigo robes, or with an indigo background as a way to indicate their spiritual wisdom and ability to see beyond the material.
    In color psychology Indigo is said to connect the conscious and the sub-conscious.  A person who is depressed should stay away from indigo as it can increase that mood.
    In healing, indigo is said to heal the bones and joints of the body.  It helps purify the blood and prevents excessive bleeding.  Since indigo is primarily associated with the energy center in the middle of the head or the throat center, it will benefit any problems in the head and throat area, especially the sinuses and eyes and glands in that area.
    Gems and stones that can help if you are weak in indigo include Lapis, Azurite, and Iolite.