Cyan. Everything was cyan.
Three men awoke in a tunnel, stretching as far in either direction as they could see, ending in black. On the ground around them, there is nothing but rocks. On the ceiling, a gooey cyan substance glowed dimly. It is the only source of light.
Flipping a coin, they head in a random direction. Stopping at leisure, dipping into the food-filled backpacks they brought for an expected camping trip that clearly didn't pan out.
The men grow tired and weak. There is no sense of night, day, or time at all. How long they walked is anyone's guess.
One twists his foot on a rock. They cannot continue. They rest for too long. There is no food.
In situations like these, certain thoughts are bound to occur, and to one man, it did. But the third man would not have any of that.
A smashing argument. One friend dies to save another. A moment later, there is a sound. A squishing sound from the ceiling. Looking up, the two men see a thing emerging. A crude cyan imitation of a human pulls itself out of the ceiling and begins walking upside down. One man drags the other in pursuit of the creature, endlessly craning his neck up. It may be their only way out.
The cripple perishes. A second creature emerges. The last man carries on. An hour, week, eternity later, the last man reaches the end.
On the ceiling, the not-men are now dozens. They are digging out. The man passes out from exhaustion.
A creature emerges. They'll make it out.
Credited to Erickson Warne-Coles