The mneumonic acronym that I learned for the rainbow spectrum colors was the name "Roy G. Biv" (Red, Orange, Yellow, Green, Blue, Indigo, Violet).
If you are talking about reflected colors (the way colors work when you are mixing paints or something like that), 3 of those are the primary colors (red, yellow, blue) and 3 of them are the secondary colors (equal parts of two of the primary colors; orange, green, violet which is sometimes referred to as magenta on color wheels).
Of the seven colors listed in that spectrum, indigo is the only one that is neither a primary nor secondary color .... in that color system.
Of course, if you are used to thinking in terms of emitted colors (mixing wavelengths of light, as from a computer or television screen), which I am, then that list is a little strange anyway. It skips one of the secondary colors (cyan), while including two tertiary colors (halfway between a primary and a secondary; orange and indigo).