I've always hated the "blue, indigo, violet" end of the English words for the rainbow. I only recently found out that the 7 colours of the rainbow in English is especially BS.

At the beginning of Newton's time, there were thought to be 5 colours in the rainbow: red, yellow, green, blue and violet. Newton added Orange and Indigo because he thought (for spiritual reasons) there should be 7 colours. There were 7 days of the week, 7 planets (known at that time), 7 notes in the (western) musical scale, etc.

But, Newton's blue isn't our blue today. For newton, the colour "blue" is what we currently call "cyan", so the 7 colours of the rainbow were really red, orange, yellow, green, cyan (blue), blue (indigo) and violet. That makes a lot more sense.

But, there really could be arbitrarily many colours in the rainbow. We could have "rust" as a colour, in between red and orange, or "gold" in between orange and yellow. We could even have a spectrum that didn't include "green" but included "lime" which is between today's yellow and green, and cyan on the other side, and no "green" at all.