Prores is a 422 codec, with an existing 4444 variation. FFmpeg comes with 3 different prores encodes: "prores", "prores_ks" (formerly named "prores_kostya") and "prores_aw" (formerly named "prores_anatolyi"). In our testing we've used the "prores" and the "prores_ks" encoders and found "prores_ks" to be the best encoder to use. It is the only one that supports the 4444 colorspace and although it may be slightly slower. The color quality of the videos produced by these two codecs was visually indistinguishable. Because of the 4444 support we've decided to go with Kostya's version of prores.
https://trac.ffmpeg.org/wiki/Encode/VFXЕсли коротко, то реверс-инжинирингом ProRes в FFMPEG занимались 3 чувака по отдельности, и сейчас в кодовой базе 3 разных реализации. На сегодня только prores_ks (бывший prores_kostya) поддерживает кодирование с альфаканалом, но работает чуть медленнее, чем безымянный дефолтный prores.