10 Small Details You Missed In Tenet

Throughout the film's runtime (credits included), the colours red and blue have a strong thematic presence as they indicate characters and objects moving forwards and backwards respectively.

This colour choice goes beyond lighting, as various characters such as Kat are dressed up in red and blue-coloured clothes during scenes that prominently feature inversion. In addition to this, there are similarly-coloured objects that play a small yet key role in the film's narrative.

Early on in the second act, the Protagonist joins Kat and Sator go on a blue sailboat in order to win the latter's trust. Here, the extent of Kat's hatred for the arms dealer is seen when she unclips his harness and sends him falling into the sea. (Un)fortunately the Protagonist saves him and this sets up Kat's involvement in the film's main events.

This scene also foreshadows Sator's eventual death at Kat's hands, as an inverted Kat is the one who kills Sator before his world-ending plans can take shape.