(1935) Becky Sharp: The first film in Technicolour.

Rouben Mamoulian directed Becky Sharp (1935), which became the world's first complete feature film produced using three-strip Technicolour technology that combined red and green, and blue film strips to create realistic colours. The innovative colour technology revealed to viewers the complete capabilities of colour movies, which transformed the entire film industry.

The film adaptation of Vanity Fair by William Makepeace Thackeray depicts Becky Sharp as a resourceful social ascender who navigates English high society during the Napoleonic Wars through her performance as Miriam Hopkins. The film received negative reviews for its storytelling and pacing, but its technical breakthroughs in colour production became its lasting achievement.

The UCLA Film & Television Archive restored the movie through original Technicolour materials to safeguard it for upcoming generations.

The movie was made through original Technicolour materials to safeguard it for upcoming generations. The movie exists as a historical milestone in cinema because it established the path for Technicolour to become the dominant film technology during the golden age of colour movies, although it remains inaccessible on modern home video formats.

References:
Wikipedia-Becky Sharp (1935)
https://en.wikipedia.org/wiki/Becky_Sharp_(film)
UCLA Film & Television Archive-Becky Sharp Restoration
https://www.cinema.ucla.edu/restoration/becky-sharp-restoration
Rotten Tomatoes-Becky Sharp
https://www.rottentomatoes.com/m/becky_sharp