Senses are how beings experience their environment and Pathfinder second edition tinkered with how senses are represented mechanically. One of the subtle but important changes to that system is the update to darkvision. This ability used to have a range increment of either 30 or 60 feet depending on the source of the ability. Now darkvision extends to the same lengths as light-based sight but why?
On Golarion many races require light to stimulate the eyes for sight. This is represented with low light and standard vision but there is a large swath of Golarion’s denizens which never see the light of day such as those who dwell in the Darklands.
Natural light does not reach this network of caverns beneath the surface but those creatures that dwell in the dark must navigate that environment. How ever darkvision is biologically accomplished limiting that sense to 10 or 20 yards was a game balance decision focused around vanilla archetypes of dungeon raiders.
If player characters could see deeply into the dungeon and know what monsters they were facing they could gain too much of a tactical advantage. Mechanical system balance serves a purpose but why would any race build large rooms or long tunnels if their sight were constantly limited to a short range? What would make sense for pawn on a table did not make sense for creatures of fantasy and imagination.
Out the door went the distance limitation on darkvision. So why not always make sure your characters have darkvision as a player? This sense has a distinct drawback, it is only perceived in black and white. This has huge implications and drawbacks if players rely heavily on this ability. Color is used to determine many variations of the environment that black and white cannot match.
Color is used to determine material types, biological signals and language signals. What is the difference between quartz crystal, ruby or diamond? In the light they are different colors but in darkvision they will only have different textures and hardness’s. If you have a generous GM, they may have different shades of grey. In nature red is often used to signal danger but in a desaturated world of black and white you would not be able to tell as well. Blue ink or black in would look just the same in darkvision.
The tasty part as a GM is defining the in between space of the mechanics. IN a world of no shadows and only black and white vision should there be gradients? What causes contrast on surfaces in your world? Once the GM has an answer that fits their table you can build great traps, puzzles and encounters for your players.
Here is a great puzzle for your party full of players with darkvision. Your group moves into the room and the thick stone doors slide shut sealing your group inside. Before you is a circle of six pedestals surrounding a central dais. On this surface are 6 stones of magnificent cut and shape. You lift them up and tell they are of different weights and each has a similar hue to your darkvision. Sculpted out of each of the pillars around the dais are spots that seem like they would fit any of the stones.
Now Players generally get the idea, the stones are keys that go in the pillars. What they may think but not know is there is a specific order to the placement. As GM you can have players roll perception checks to see if they can tell any minute details. Feed the party tactile information about each material but not their colors since everything is in black and white. If the party places the stones incorrectly maybe a trap triggers or a monster drops into the room. If the players light up a torch suddenly the puzzle is easy, each pillar is the same color as a stone. Give them a hero button and move to the next encounter. This is a simple puzzle that light-based sight quickly resolves. Do not be afraid to reverse the idea. Your heroes must put the light out and rely on the PCs with darkvision for the clues needed to avoid the trap.