Paleness is a quality of lacking color, particularly when it comes to a person's complexion. Your friend's paleness might be the first sign that she's not feeling well.
You can talk about the paleness of anything that doesn't have a normal amount of color: "The paleness of those peaches makes me suspect they're not ripe yet." This noun is most commonly used for the pallid face of a shocked or sick person. A patient's paleness will worry an attentive nurse, and your little brother's paleness will make you realize the movie you're watching is too scary for him.