warning tissue
These are facial tissues that are differently colored from the box's primary supply. They are located towards the end of the tissue box's lifespan. These serve as a warning to all users that the box's supply will soon run out and that it's time to purchase a new box of tissues.
"Dude, the next tissue isn't the same color!"
"Yeah, that's the warning tissue, you better get a new box! This one's almost out."
"Yeah, that's the warning tissue, you better get a new box! This one's almost out."