Indigo is sometimes discounted as being one of the colors. "Rainbows have six colors" was what I was taught growing up in the USA years ago, and it was only when I was older I found out indigo became the seventh. I'm unsure how prevalent this is today.
The seven colors of the rainbow where used by Isaac Newton because he preferred the numerology associated with seven. Yukari is correct insofar as in common English usage, indigo is considered a shade of blue or purple rather than its own primary color.
Originally, colours had no border.In Japanese, it’s seven colours.How many colours are there in a rainbow?Ehh...There’s also other languages that mentioned four or two colours.So there’re many correct answers, aren't there...But in English it could be six colours.Isn’t it… seven colours?Umm… right…and wrong.