Davriel is UB just like Brandon's favorite colors.
Tacenda is probs WG before the entity but I guess we could see her Abzan.
I hope both of the demons get cards too.
As for color identities here, I'd mostly agree with you. I see Davriel as mostly black, but with a blue element to him. I could see him printed under mono black, UB, or even--under just the right circumstances--esper. (He believes in the structure of laws and society, though admittedly mostly as a thing for "other people.")
Tacenda has a strong red streak to her--in fact, my initial concept for her was a mono-red character, but one who expressed the red aspect through music, song, and passion. The entity inside of her is green, however, and the white/green nature of her society has had a big influence on her beliefs in fate, the needs of the many, and that sort of thing. So I'd make her RWG.
I was considering the same colors for both of them as well. Definitely black and blue for Davriel. I was personally leaning towards Esper, however. Like you said, he does believe in structure of law and society. Also, his ability to summon weapons seems like a white effect to me.
Well, he stole the weapon-summoning spell. I was told I could have Davriel steal and use spells of any color, so long as it was painful for him, and it was clear he was using them as part of a theft mechanic--meaning he only had access to them for a limited time.
I personally thought he was grixis. I know I am wrong but, he is a demonologist, and I know demons are mostly black, but secondarily red. As well, most devils are also red. As for stealing powers is also very dimir, so I understand that. I would like to know what is the white mana coming from?
He doesn't use white mana--he can use black mana to cast white spells, if they are stolen. However, there's not a lot of red in Davriel, despite his fondness for devils. He's not emotional, or artistic, and is more about planning and forethought than intuition.