ROY G BIV

Sooooo. . . I have this issue with whoever came up with ROY G BIV.  I have always loved rainbows and colorful things.But what is with the indigo?  Three primary colors, three secondary colors and. . . one tertiary color?  Why? If indigo is in the rainbow, then orange/red, and yellow/green should also be there, shouldn't they?  Was indigo just added so that they could have a cute little acronym