I don't remember if I noticed this on my first analysis or not, but I just registered that the Monokubs get their own little portrait after they die too. Major spoilers for DRV3 in case anybody cares.
Except they get the pink X while the participants are getting red Xs - which is the only Red X we see in the mainline series.
This time is the first time in-trial we see red.
When Red Blood is used elsewhere in the series, it's to drive home the reality/significance of something outside the Killing Game. Sato's Crime Scene in Twilight Syndrome Murder Case, and the room where the Student Council killed each other in DR1.
Red blood being on the cast portraits while the Kubs get the stylised in-universe "fake" pink blood does a good job of implying the darker out-of-game reality in a way that you can easily miss.
Although the more I play the more I'm like "wow this story wasn't as good as I remember", the more I'm like "at least in-universe this was supposed to be the "we're running out of ideas" season and so putting all the effort into foreshadowing the twist/justification for the weaker story at least makes me respect it.
