This sounds a good bit like Brain Plague, part of the Elysian Cycle by <PERSON>. Published in 2000. The main character, <PERSON>, is a starving artist in the far future on another planet. Desperate for funds and health care, she signs up for a kind of surgery to host a colony of microbes or "brain enhancers" who happen to feed off of arsenic, azetidine, or "ace" in the bloodstream. Medical procedure like an implant.
<PERSON> learns that her new occupants are chatty, friendly, and worship her as a god. Because of their short lifespans, she has to attend to them, or even the kindest colonies will try to overthrow their host and take what they can get from the bloodstream. They appear in colors that not every race can see, twinkling and shining. They can manipulate dopamine and at one point try to convince her to let them dominate her body in exchange for a high (I think it was called "endless light"). Lights and colors.
Because of social and political issues, an elven race tries to manipulate carriers/hosts by training the microbes to attack the host and slowly kill them, but not before infecting others. 322
The infected "vampires" are drug addicts for arsenic for the microbial colonies. Drugs.
The community of carriers are more than just hosts, to speak to your point about detectives. <PERSON> ends up learning about other hosts through various chats with microbe transfers and begins to help a friend who is much more deeply involved in the politics. There is a network of humans and other races trying to keep the use of the brain enhancers safe, and between the police, user agents, and medics, they suspect a sinister plot. Detective work.
This image of the cover shows the main character with red hair. Color and the brilliance of light was a big theme as different races could see different parts of the spectrum; she even named her "people" after colors. The microbes can also "flash" at other hosts through the carrier's eyes. They communicate with hosts quite conversationally and twinkle as lights in her viewport. Communication and light.
Related: Novel with a female protagonist who can see ultraviolets 322