The Scottish musician Donovan, a good friend of The Beatles, made a key contribution to this song, coming up with the line "Sky of blue, sea of green."
After he got the idea for the song, Paul McCartney dropped by Donovan's place and asked him for suggestions in hashing out a verse. In our interview with Donovan, he explained: "He already had those words to the song, but he seemed to have a hole in the song. So I took his words and turned them around for him."
This line is Donovan's best-known contribution to a Beatles song, as it's the most concrete, but it was simply adding a line; he takes more pride in other Beatles songs he influenced on their shared musical journey. In February 1968, he joined the Beatles on their retreat to India, where he taught McCartney and Lennon the "clawhammer" guitar technique, where the picking hand strikes the strings in a downward motion with the back of the nail. McCartney used this technique on "Blackbird
," and Lennon used it on "Dear Prudence
." He also helped Lennon with another song written in India, "Julia
," which John wrote about his mother.