The video embedded in the Vox article is worth watching. I didn't realize the sepia tones in O' Brother, Where Art Thou were a digital effect using techniques that were very new at the time. The film was digitized, color altered and then printed back to film for distribution.

I've uploaded a before and after shot of the baptism scene here: https://imgur.com/a/yAugAV0

Painting With Pixels (O' Brother, Where Art Thou)

https://www.youtube.com/watch?v=pla_pd1uatg

Description: This short video about the Coen brother's film 'O Brother, Where Art Thou', the first feature film to employ a full digital colour grade.