Duotones are images that only display in two colors.
Like most visual techniques on the Web, duotones come from the world of print. In print, the more colors you use, the slower the production time and the higher the cost, so duotones were often an economical alternative. Duotones can also improve efficiency on the web by enabling the creation of cool-looking images with small file sizes. Duotones are made by first creating a grayscale image and then overlaying it with a different specified color.