CMD of F and 1O SMC Cepheids. The boundaries of our empirical IS are shown as red empty circles. The median intrinsic colors of each bin are shown as black empty circles. The upper and lower parts of the fitted red and blue edges are shown as black solid lines. The edges of the wedge-shaped IS are shown as dotted lines. The periods for these stars are shown with a color gradient. For 1O Cepheids, periods were fundamentalized. The overplotted dashed lines represent constant periods.