Nobledark, you mean. It's Noble/Grim and Bright/Dark.
Noble/Grim are about heroes. Can heroism work? Are heroes changing the world, or just drops in the bucket raging against the machine? "Nobility" refers to the nobility of heroes (and villains!). It's not necessarily the case the everything is great: if the "Noble" refers entirely to evil people, it could be a pretty shitty world. But in Warhammer 40k for several millenia, even absolutely massive, huge-ass heroes can give every thing they can and it just won't matter. A spacemarine is a god relative to humanity, but there are millions of them! And they can't save the imperium! And think about Warhammer videogames where a villain wants to blow up the planet: it's incredibly telling that a video game is allowed to invent and just *blow up* an entire planet depending on a player's actions, because the destruction of an entire planet is just, a drop in the bucket. Destroyed a Forge World, demons? We have reserves. Basically in a noble universe, heroes have the ability to change the entire setting if they are good/evil/strong/fast/willful/noble enough. Grim says "This is life! Is it awesome? Does it suck? Doesn't matter! Forces beyond anyone's control are all that matter."
Bright/Dark are more about the setting as a whole. A bright world is full of wonder, technology, advancements, all that jazz. Healing is possible, resurrection may be possible in the setting (and resurrection without drawbacks like losing your soul). Society advances. Perhaps there's some type of golden age going on. Dark worlds are the opposite: life is poor, grey, rusting, and unimpressive. If you lose an arm, your not getting it back without sacrifice. People don't go on adventure for the sake of adventure, and there's not that much you would want to go see and do anyway. Life is Hobbesian: poor, brutish, nasty, and short. Perhaps the world is sinking into some sort of dark age as well; it's not only that things suck, but maybe it didn't always suck either. Imagine LoTR's universe: all sorts of wonderful amazing things used to exist, and now the world is just so much colder and grey.
If heroes actually can change anything, new warhammer 40 is nobledark. If not, GRIMDARK.
EDIT: Speaking of BRIGHT, I noticed TotalBiscuit did his thing on Dawn of War 3. He's a massive warhammer fanboy, so apparently he broke some sort of embargo he had on SEGA or something to do the review. He's not impressed. He doesn't buy into the whole MOBA thing, but he's pretty damn critical.