The yellow sheet of paper symbolizes Tom's internal conflict of personal happiness versus ambition. Tom is faced with the drive to climb to the top of his corporation through innovative thinking and forward-projecting projects. At the same time, Tom has a lovely wife whom he admires and enjoys; so far, there is no disharmony between them, though Clare recognizes, like the clear voice of reason, that Tom works too hard. The yellow paper importantly symbolizes what might be Tom's wasted life:
He thought of all the evenings he had spent away from her, working; and he regretted them. He thought wonderingly of his fierce ambition and of ... the hours he'd spent by himself, filling the yellow sheet that had brought him out here. Contents of the dead man's pockets, he thought with sudden fierce anger, a wasted life.
The color yellow itself symbolizes many things in many cultures. In this story, yellow symbolizes both sides of the internal conflict facing Tom. Yellow's versatile color symbology represents both (1) danger and death and (2) joy and wisdom. Each represents half of Tom's internal conflict: danger and (near potential) death is what ambition gets him while joy and wisdom is what pursuing true happiness gets him.
That the "creased yellow sheet" goes out the window symbolizes the divide Tom's internal conflict is creating between him and his true happiness embodied in Clare. With Clare, Tom is safe and warm, making breathe patterns on the interior side of the glass. While pursuing the yellow sheet and holding it between his teeth (like a dog with a bone), Tom is locked out of his home, looking in from the cold with "his forehead pressed to the glass of the closed window." The yellow sheet symbolizes this divide (as does the window).
suddenly, the strain slackened and ended, his chest touching the window sill, and he was kneeling on the ledge, his forehead pressed to the glass of the closed window. ... he stared into his living room--at the red-brown davenport across the room, and a magazine he had left there; ... and at his papers, typewriter, and desk, not two feet from his nose.