Everyone is familiar with Gertrude Jekyll’s dislike of magenta – “malignant magenta”, as she called it. Many other writers of her period were equally dismissive, such as Alice Morse Earle, who said that as she glanced back through her writing on the subject, she felt the word “made the black and white look cheap.” (Kellaway: 93-5)
Seven planets, seven notes in the Western major scale, so of course seven colours in the rainbow. Isaac Newton even felt that there were similar intervals between colours and notes. (You can hear the tones and see their associated colours on YouTube.) Presumably indigo fitted in well between blue and purple, but most modern scientists who study light consider the spectrum to consist of six colours, the primaries and their complementary colours.