John Lennon found the Spanish location quite boring, so Sir Ringo Starr flew out to keep him company. However, Lennon also found working on the project a refreshing change from his increasingly arduous and isolated musical career with The Beatles. Namely on set, Lennon was just another cast member, and a relatively minor one, who could interact with others largely as a normal person with the crew. As such, Lennon insisted on attending the production every day even when he was not required on set as a learning experience.
The intended concept of the movie was that archival black-and-white footage of each battle was to be tinted specific colors. The soldiers who died in that battle continue to accompany the unit silently with their uniforms now the color of the battle in which they died. However, when the final answer print was sent from the principal lab in England to the labs in Los Angeles subcontracted out to make the release prints, the Los Angeles labs assumed that the tinted footage was a mistake, and graded it back to black-and-white without notifying anyone until the whole batch was struck. The version of the movie shown on "Retroplex" from Starz has the battle scenes tinted different colors and the soldiers who die in a battle become that color.
John Lennon recorded all post-synchronization work for his character in the movie on December 28 and 29, 1966, at Twickenham Film Studios in London, England.