For the buddy cop/serial killer au:
White is the victim that gets attention drawn to the case. Not the first. There were ones before them, but didn't get prioritized by the police force since the victims weren't anyone notable. White's in reality probably the fourth victim and Green did kill White with the goal of getting attention in mind. Some form of celebrity. Little to say otherwise.
Blue is the medical examiner/forensic patholothogist who is probably one of the only competent person on the investigation but is limited in their ability to solve anything since their role is mainly concerned with autopsies. Has a dynamic with Red that when evidence is mentioned around them, they will suggest a more reasonable possiblility/next course of action that causes Red to get insecure and dismiss what they're saying and double down on the original theory which just ends up hindering progress. Breath of fresh air for Purple to have someone who's actually helpful on the team. Killed for getting too close to the truth.
Lime is the crazy conspiracy theorist who nails the killer's identities on the first try. Probably in the police station for something unrelated, takes one look at Green and clocks them immediately. The knot Green used to tie their shoes is the same weirdly specific knot used to tie something at the crime scene or some other reason that's hard evidence to Lime but sounds batshit to everyone else. Green never truly thinks Lime could ever actually be convincing enough to get them arrested, but they are doing a double take at their more minor habits that could make them suspicious to someone more charismatic. Eventually decides to take matters into their own hands by confronting Orange who they also suspect and is killed by Orange.
Orange is hr representitve for the department and a copy-cat killer. Or at least attempted copy-cat killer. Hears everyone is the department going on about how cunning the killer must be and thinks 'They're not smart these guys are just dumb' In fairness they are, but Orange is dumber. So as they're actively planning their own murder in their office, Lime barges in, starts throwing all these accusations at them, then Orange thinks 'might as well kill them now. perfect opportunity to take out someone who's already onto me' and kills them... with literally the entire department outside the door since they were chasing down Lime. They'd be able to claim self-defense if they didn't monologue their entire plan to Lime (within earshot of everyone) before killing them. The previous murders are assumed to be by them. Orange goes along with it since their fucked anyway and those murders make them look smarter. And if we're sticking to canon, they're executed. Once the murders start up again, Purple concludes Orange falsely confessed.
Cyan is an officer on the force. Hired for bringing good vibes to the team rather than merit. (sweet as they are, they're better emotional support than a crime solver) Befriends Purple once they're on the force, (Red totally isn't jealous) and spends a lot of time attempting to befriend Black with little luck. Discovers a pattern in the murders, but struggles to articulate or even fully understand what they've discovered. Serves as Purple's anchor until being murdered by Green which deepens Purple's growing obssession with catching them because now it's personal.
Yellow is the co-owner of a local restaurant with Brown within the area where the murders are taking place. They're a vocal critic of the police force and serve as a source of pressure for them to solve the murders. Probably ends up organizing some kind of neighborhood watch system / civilian patrol once the murders go on long enough They eventually end up being framed by Green and are scapegoated by the police force who at this point is desperate to have a name to connect to the murders. And if we really want to go by canon, they end up executed for it. Green stalls the murders during this time to give eveyone a false sense of peace (for the fun of it obviously) before starting the murders right back up once Yellow is dead. Public trust in the force was strained before, it tanks after they realize they got the wrong person. With Orange it was reasoned 'oh, everyone saw them kill Lime and they confessed to the other murders it would only make sense for the police to convict them' but Yellow went down pleading their innocence the whole time. Red has to live with this on their conscience. Purple too. They could've stopped this.
Brown is the co-owner of a local restaurant with Yellow within the area where the murders are taking place. Also a critic (though a little less vocal) of the police force and serve as a source of pressure for them to solve the murders. Especially after Yellow's death. Contrary to canon, I don't think Green would kill Brown. The goal in canon was ultimately 'kill everyone' in this A.U. it seems to be more 'have fun with and mentally torture those involved'. They would leave Brown alive to grieve Yellow and to leave the force a living, breathing reminder that they murdered an innocent person.
Black is a forensic geologist working on the investigation. They're one of, if not Green's final victim before finally getting caught. They'd piece together crucial evidence that could point to the killer's identity using the notes Cyan left behind.
And you've got the other three on lock, lol
GAMMA, I AM TAKING NOTES. I LOVE THIS SO SO MUCH. Esp about White being the first case that triggers an investigation bcos they're the first high-profile victim that forces the police to investigate. White's death put pressure on the police, bcos of their family background and their fanbase + the media attention their death got. Typical.
And since no one knew about the other cases, I bet it was Purple who discovered the link. They probably have one or two of the original killings that already appeared connected, so by the time Red's pathetically pleading for them to help, Purple already has the info of these two cases to match to White's. Maybe a family member of the other victims approached Purple, bcos the victim went missing and the police weren't doing anything.
I like the concept of Orange being a wannabe killer. Maybe they're a copycat of Green. Which initially screws the case up, bcos Red insists Orange must have killed White and the others, while Purple says the MO doesn't match at all. Until Cyan dies.
I really like how you fleshed out the concept to match canon so well, UGH, THEY'D REALLY ALL FIT IN A HARDBOILED MYSTERY DRAMA. And how cocky is Green, to start killing people that close to their case. They can't help but leave bigger and bigger clues for Purple to find, only for Purple to get sidetracked bcos an egotistical murderer who needs constant attention or else they crash out, with intimate insider knowledge of the case? Someone who'd benefit from a copycat killer taking the fall for the other murders?
Of course it's Red. Never mind why Red would start killing again so soon after Orange was blamed for their "crimes".