The aim of this research is to develop new mordants and a wide range of colors for natural dyeing of silk. In this study, two new erbium(ΙΙΙ) complexes, [Er(AO)(H2O)7](OAc)3 and [Er(AMH)(H2O)6](OAc)3 (AO = acridine orange, AMH = azomethine-H, and OAc = acetate anion), were synthesized, characterized and used as a mordant for the natural dyeing of silk yarns with an aqueous extract of green walnut husk. The mordanting method and the effective dyeing parameters including the quantity of the dye material, the pH and temperature of the dyeing bath, the mordant concentration, and the dyeing time were carefully optimized. The colorimetric properties (L*, a*, b*, and K/S) and color fastness characteristics (light and wash) of the dyed samples were also investigated. The mordanted silk yarns with synthesized erbium(ΙΙΙ) complexes were compared to unmordanted dyed samples as well as mordanted dyed samples only with simple salts including starting erbium salt (erbium(ΙΙΙ) acetate), stannous chloride, and aluminum potassium sulfate. The results indicated that the dyeing of silk in the presence of [Er(AO)(H2O)7](OAc)3 as the mordant provides the highest color difference, whereas the treated silk with [Er(AMH)(H2O)6](OAc)3 shows the most color strength value (K/S). Furthermore, all of the mordanted and unmordanted samples exhibited proper color fastness characteristics.