No...it's seven in English too, if I'm not mistaken.

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.

Indigo is still used as a color of the 7 in america. At least my science classes have been using it frequently for light refractions.

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.