Remember kindergarten, and all the lessons on sharing? You're going to put them to use now.
Have a character share something, whether it be lunch, a secret or a T-shirt. The sharing can be the focus of the fic, or merely in the fic to emphacize something, such as the closeness of the relationship.
Challenge Two: Black and White
This is a companion challenge to last week's Shades of Gray challenge.
Sometimes people have very "black and white" views on things. Focus on absolutes. Tell a tale of contrast, good and evil, two extremes. Or use the colors literally.