Kuroiwa Cases
93-95: Special Cleaner. Red first appeared with something inside a closet...? Hard to describe, but worth noting later Hama saw some humanoid thing inside the closet but that panel wasn't red. Red appeared with bodies, and overlapped with injuries on a single page in 93. Kuroiwa had red spots around his hand and face in one panel on that page. 94 notably ended with a red page when their other team member said they'd stay, and he ultimately died. So the red may have indicated he was already cursed.
99-101: Cotton Glove Dropper. The red didn't appear much until the final chapter, and would overlap with the giant sky fingers smushing people's heads. One smushing panel had red specifically around a person's eyes. Interestingly there were panels that were red with just speech bubbles. First someone asking "What kind of job is this?", and then "Stand up" to a guy who was in shock after seeing a person get killed. Next time was them yelling the name of another person who got squashed.
Back then I theorized that the hands would target anyone who paid too much attention to the ritual (you can see my comment on 101 for more details), and the panels that were red involved people directly acknowledging what was happening. So, I think the red was an indicator of the curse/danger tied to the sky hands.
111-113: Proxy Grave Visitor. That one was about visiting a grave haunted by spirits of a massacred village. Red first appeared on a panel with someone's body at the end of 111 surrounded by spirits. Then 112 had it appear in the background of a character mentioning "Er, chief... About that video I took..." Said video showed the spirits, but the video wasn't red. Red appeared in the final pages of 113 showing a lot of bodies and the spirits smiling and praying. So, red seemed to indicate the spirits about to or already taking action.
172-174: Ship Crewmate. Opens up with a guy's eyes bleeding red, and red margins with white gaps shaped like a cross. Red next appeared highlighting bits of a body being eaten by seagulls. It was also in the background of the same guy flashing back to various atrocities he committed. The red interestingly highlighted someone's face with a red cross, and the organizers thought he was "the child of god" whose sacrifice would guarantee eternal paradise. I think they saw the cross, though that guy wasn't the chosen one. Then everyone BUT Hama ended up with bleeding eyes. Chapter ended with someone finding the inverse of the cross, which canceled out paradise. Final page was red with the text "TOO BAD EVERYONE. HOW TRULY UNFORTUNATE."
184-186: Shrine Guides. This shrine is all about people with curses, people go there in hopes of being purified. Red first appears with some giant overlooking the path. It then appears with an image posted online of Kouta, a boy who'd gone missing. The red panels after that were all about a man named Hoshina seeing Kouta's spirit and getting stuck on the path, and it's revealed he killed Kouta and was cursed by his spirit. He went to the shrine to try to get purified, but was spirited away instead.
One important note: I don't think they got that job from Kuroiwa. He did appear at the end of the first chapter looking at a text from Hama's dad, but I think they may have been directly hired by the shrine.
203-205: Auction House. Only appeared at the very end, and only as notes from the Manga One Editorial Department. So, it broke the fourth wall.
207-209: Human Billboard. Interestingly, the red only appeared in the last chapter. It came up in relation to an idol for the Black Tokyo Party, who worship "the Black Sun", and related to gathering a tribute for their idol. Had some red kanji say "TRIBUTE COMPLETE" as people floated up, and ended with the Black Tokyo Party spokesperson talking to the idol. Interestingly his panels were white but the page had red margins, and the page with the idol was all red.
And... That's it. Those should be all the red cases. This was very interesting. If anyone wants to reread those cases and add their own thoughts, go ahead!