I'm afraid that Darkvision doesn't give colours under the D&D 5e rules and I guess they are on a Darkvision YES/NO switch which stays on no matter what the external lighting.
There is an argument that, in a primarily visual medium like a CRPG, that part of the rules/lore could be dropped for a more colourful experience, but I like that the Darkvision spectrum is Darkvision and not just somebody turning up the lights.
As for what I see as your actual point (about Darkvision not being normal vision when others have torches), yes, I agree. It might have been overlooked by the developers and could count as a bug.
Possibly a matter of interpretation? Not sure though. I had interpreted seeing in dim light as if it was bright light to mean colours were normal, as they would be in bright light. Darkness as if it was dim light would be the shades of grey.
A monster with darkvision can see in the dark within a specific radius. The monster can see in dim light within the radius as if it were bright light, and in darkness as if it were dim light. The monster can't discern color in darkness, only shades of gray. Many creatures that live underground have this special sense.