According to Wikipedia, there is no uniform agreement on this color's place in the visible or electromagnetic spectrum. More than a dozen combinations of primary blue and purple are named "indigo." The color standard might be the dye derived from the plant Indigofera tinctoria from India, and also from the woad plant, Isatis tinctoria, but the pigment is now mostly produced chemically or displayed on computer screens.
What color is an Indigo Bunting? Actually it is black, and the blue color is not due to pigment but rather the reflection and diffraction of light from its feathers. Given the vagaries of light, shadow, reflection, background, camera ISO and shutter speed, and the effects of post-processing and the computer screen settings of the photographer as well as that of the viewer (or the ink used in printing), photographic images provide a whole range of answers. Following are some of the variations on the indigo theme.
The head of the adult male Indigo Bunting in breeding plumage appears deepest in color, approaching true indigo:
Bright Metallic Indigo on blue:
Navy Blue (low light and distance contribute to this effect):
Dull Blue, also the effect of low light or backlighting:
Greenish Blue (I'm not sure whether the tint is real or an artifact of post-processing):
Slightly blue (Adult male, molting or non-breeding):
Not blue (probably an immature female):
Not blue (I believe this is an adult female because of the richness of its brown back; older females usually show some blue on their wings and/or tail):
Partly Blue (an adult male in winter):
Blue with green companion (winter adult male Indigo Bunting with female Painted Bunting):
Belligerent Blue (standing up to an Eastern Kingbird that tried to interrupt his singing):