1. A method comprising: obtaining video data depicting a head of a user; determining a set of line segments of pixels of the video data, wherein each line segment in the determined set of line segments intersects an upper contour of the depicted head of the user; grouping at least some of the pixels of at least one of the line segments in the determined set of line segments into three sets of pixels based at least in part on respective color data of the pixels, wherein the three sets of pixels include a skin-pixel set, a hair-pixel set, and a background-pixel set; and updating a user hair-color model based at least in part on the hair-pixel set.