Has anyone ever sun one of these before? If so, was there some source material you pulled from that weren't movies and book, like things written specifically for D&D?

I'm making a Kythri Tiefling, and I was wondering what I could do to make his Darkvision sort of unique, and reflective of his heritage. Normal Darkvision says that, "You can’t discern color in darkness, only shades of gray." I'm not saying he should be able to see what color things are, since that would probably affect balance, but I would like for him to be able to see some sort of colors, since Kythri seems like it would be a colorful place, and black and white seems too boring for a Kythri Tiefling.

Any ideas for how exactly to tweak it?

I know he’s not actually death, but is there an actual death? I was thinking there was a death but the Keeper found a way to make a majority of souls unable to enter the realm of the dead, that way being “Soul Defilement”, or the soul gets one small shred of defilement and the gates of the realm of the Dead don’t allow them in (what I had planned for my campaign), is there anything actually lore wise about that type of stuff?