"Wait, is that... chalk? How could you betray us like that?!"
— Yellow Crayon, "Your Opinion is Invalid"
Strict rule follower acts like a teacher's pet and can come off as snooty.
As of OSO 1, it's established that Yellow and his fellow crayons reside inside of Crayon Box. He is eager to explain parts of the show, even interrupting Crayon Box to do so.
As of OSO 1, none of the contestants knows what his purpose is. He acts defensively when Crayon Box was asked a question.
Yellow Crayon is a light, rectangular, yellow crayon with an unscathed top.
Episode 2 - "Your Opinion is Invalid"
He is later seen at a podium at the elimination, handing Crayon Box the slideshow controls.
After Crayon Box moves the chalkboard into place, he pops out of Crayon Box and scolds her on using chalk instead of using them to explain challenges, in which Crayon Box reluctantly allows him to explain the challenges, and jumps onto Green Crayon.