This was a manga I found on myreadingmanga but no it was not a hardcore yaoi or anything,
actually I distinctly remember reading it and thinking "huh is this even yaoi?" but anyways -

I'm pretty sure the main character was either a detective, cop, or someone related to crime/solving mysteries, and then along comes a kid that can see the color of people's auras?

And the kid specifically mentions that the main character's aura was (I think) completely transparent or something like that.

There's another guy that's either the main character's ex lover...

And the detective adopts(?) the kid, or at least takes him in...

I think the title was one word. The tone of the manga was fairly serious.

I remember really liking it, but completely forgot about it, I think I read it about last year or 2 years ago.

Is it Op: Yoake Itaru no Iro no Nai Hibi? The title's often abbreviated to just "Op," and while the mangaka is Yoneda Kou, it's not actually BL and is being serialised in a seinen magazine.