You most likely only looked at one area. Its dark (souls) for sure but in just the first game there are plenty of areas that arent just gray. Darkroot garden and Basin as well as valley of the drakes are all a nice green, anor londo is just beautiful in general, Lost Izalith and the Demon ruins have reds and oranges everywhere (lots of fire and lava) and more. That is if we are talking about ds1. For the other games I would have to give different examples because none of them are in the same world.