Other people have pointed this out - colorism is evident in Trese, whether or not the creators/artists intended or realized it. I'm just listing down the characters and suggesting what the creators/artists were trying to do.
The palest characters are more or less Alex, Crispin, Basilio, Nova Aurora, Coreen, and the Emissary of Ibu (IMO, they also have more Eurocentric features). Cpt. Guerrero, Lt. Tapia, the Bagyons, Maliksi, Dominic (the hot aswang leader), the hot manananggal leader, and the hot diwata all seem to have light brown skin. (With Team Trese, their pale skin isn't just due to lighting: for instance, when they speak with Jobert, Hank doesn't seem any paler in the dim room while Alex and the Kambal are almost vampiric.)
Hank, Alexandra's family (parents, brothers, and grandfather), Datu Talagbusao, Ramona, Hannah, and Ammie have more kayumanggi / brown skin tones. Additionally, Marco, Raul, Ibwa (and his crew), Gen. Villar, Mayor Santamaria, and many minor characters, particularly the cops in Cpt. Guerrero's precinct, Santamaria's inmate-henchmen, and the abducted poor people (in Episode 1) all have brown skin tones.
It seems that the characters' designs were meant to (1) be unique and add to their characterization, and (2) loosely reflect their social class or hierarchy.
The former is more evident in major characters (i.e. Alex, the Kambal, high-ranking supernatural creatures); the Kambal are even teased for wearing amerikana in hot weather. (It also seems like the supernatural creatures who look more human wear more modern outfits.) That being said, they could've made Alex and the Kambal more kayumanggi while they were at it.
(I don't think the Kambal's demigod status accounts for their skin tone, considering that Datu Talagbusao definitely has brown skin, and Ramona is likely tan as well [even though all her scenes were in grayscale]. Similarly, even when she was younger, Alexandra is notably paler than her family.)
The latter is more evident when comparing the appearances of less prominent characters: Nova Aurora and Coreen are both very pale and likely belong to the middle or upper class, while Raul and the abducted people in Episode 1 represent those who belong to the lower class. Additionally, Ibwa and his crew, and the cops in Cpt. Guerrero's precinct have darker skin.
These likely resulted in more of the protagonists and "good guys" having lighter skin and more of the villains and minor characters having darker skin. For example, Ibwa and Xa Mul (and the other aswang that Alex fights in Episode 1) have darker skin, as well as the abusive cops in Cpt. Guerrero's precinct. I don't know if the creators/artists realized the implications of that.
Also, if they could combine plots, characters, and themes from different cases/storylines in the komiks, couldn't they have also given more of the major characters brown skin tones and more Filipino-looking facial features?
Fair/pale skin and Eurocentric features are still favored while dark skin is still discriminated against. Historically speaking, this resulted from centuries of colonization; we even have terms such as mistiso / mistisa (from Spanish mestizo, half Spanish and half Filipino citizens) which refers to fair-skinned Filipinos. Moreno / Morena, also of Spanish origin, refers to dark-skinned Filipinos.
Some modern-day manifestations: many actors and actresses are pale, there are many advertisements for whitening products, there are many clinics offering whitening treatments/services, many people with dark skin have been teased/judged for it at least once, and people with pale skin have been told "maputi lang" ("they're just fair-skinned"). Many Filipinos are also racist towards black people and even Aetas (who are some of the indigenous peoples in the Philippines). More generally, there is discrimination against people who aren't conventionally attractive or have Eurocentric features.
Colorism is still an issue even though, in the recent years, there has been a positive shift in attitudes towards darker skin as more actors, actresses, beauty queens, and other popular figures retain their natural brown complexion and/or actively promote being moreno/morena. Things might be getting better, but I don't speak for everyone (especially those who have been bullied for having dark skin).
Not everyone will care about skin tones (particularly in a work of fiction), and some will argue that light-skinned Filipinos do exist. However, there will be Filipinos (including me) who would feel better represented and/or even more comfortable with their own appearance if the characters (especially Team Trese) got brown or darker skin tones. (I personally wouldn't mind if they implemented that in any succeeding seasons.)
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