Indigo should not be consider an official "color of the rainbow". The colors of the rainbow are generally considered red, orange, yellow, green, blue, indigo, and violet. The three primary colors are red, blue, and yellow. Green, orange, and "violet" are secondary colors (made by mixing two primary colors). Indigo, however, is not. It's a tertiary color (made by mixing a primary color with a secondary color).
Furthermore, if you look at a rainbow, the violet and blue side doesn't take up any more space than the orange and red side.