Today we're taking a trip through the rainbow as we look at the etymology and origins of the names we use for colours. For simplicity, we're going to start today with the classic "rainbow" colours, which Sir Isaac Newton dubbed the spectrum, from the Latin for "apparition". The term later became used to reference the visible light split through a prism, another Latin word meaning "sawed", which originated as the Greek term prisma.
The first colour of the rainbow has origins in several languages and unfortunately can't be traced back to one single language. The word red was written as rēad in Old English. In fact, the British surname Reed is from the Old English for red, and is pronounced in a similar manner to how it was said before vowel shortening occurred in Middle English.
Before Old English, the word was rauthaz in Proto-Germanic, from rewdʰ, a Proto-Indo European (PIE) word. As a result of this origin, a large number of languages have similar words for the colour.
The word, colour, and fruit called orange, is often subject to a large degree of debate. While many people claim that it is one of the only words that rhymes with no other word, this is not actually true. The word sporange, a sac where spores are made, is one of the few words that rhyme with it that isn't a proper noun.
Rhyming aside, there is also a debate as to whether the fruit was named because of the colour or whether the colour was named after the fruit. Etymologists consider the colour to be named after the fruit since the word's origins are from the Sanskrit word for the tree. नारङ्ग or nāraṅga made its way into Persian as نارنگ, or nārang, before reaching European languages.
While the word nārang remained fairly true to its roots in a number of European languages, when it reached Old French it is thought to have lost its initial "n" due to rebracketing, whereby the initial "n" was thought to be part of the indefinite article "une" so that "une norenge" was heard as "une orenge".
Yellow has an interesting etymology that is similar to that of the colour red. Yellow's roots begin with PIE languages. The root of yellow in PIE has retained the same root as yell for several millennia, as both words originate from the PIE root gʰel-. This shared root has resulted in a number of European languages, particularly the Germanic languages, having similar words for yellow. The words for yellow in Dutch, East Frisian, German, Swedish, and West Frisian all have similar origins.
The term ended up in Proto-Germanic as gelwaz before it became geolu in Old English. This Old English term gave us the word we use today for yellow. However, it should be noted that in Middle English, the term also referred to colours and tones that we wouldn't consider yellow by today's standards, including a number of blue and grey colours.