Color sampling for displaying content items

An online system is configured to provide content items to users. The content item includes an image, and is displayed with a color block or colored border as an accent to the image. The color of the color block or colored bordered is dynamically selected based upon the image included in the content item, and is selected using a waterfall selection method comprising a sequence of different selection methods, each selection method configured to select a color based upon a most common color of the image that satisfies a respective set of selection criteria or determine that no color meets the respective set of selection criteria. The selection methods may comprise a dominant color selection method wherein the selected color corresponds to a most dominant color of the image, and a complementary color selection method wherein the select color is a complement of a dominant color of the image.