Recently, the interest in natural dyes in the textile industry has risen due to their nontoxicity, less polluting, and environmental-friendly. To obtain varying color/shade and to improve the color fastness characteristics of any natural dye, selecting new mordants is an important part in the development of natural dyeing of textiles. For this purpose, metallic salt mordants and biomordants have been used in natural textile dyeing. To the best of our knowledge, There is no research on the natural dyeing of fibers using the rare-metal complexes as a mordant in the literature, so far. In this study, two new erbium(???) complexes, [Er(AO)(H 2 O) 7 ](OAc) 3 and [Er(AMH)(H 2 O) 7 ](OAc) 2 (AO = acridine orange, and AMH ¯ = azomethine-H, and OAc = acetate anion), were synthesized and characterized. The complexes were used as a mordant for the natural dyeing of silk yarns with green walnut husk extract for the first time. 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. Via optimizing, the optimal conditions include the dye concentration of 15 g in 200 mL of distilled water, mordant concentration of 5% owf, pH = 3, dyeing temperature of 90 °C , and dyeing time of 50 min. Furthermore, the post-mordanting method was selected as the best mordant technique from the three different mordanting methods. 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(???) complexes were compared to unmordanted dyed samples as well as dyed mordanted samples only with simple salts including starting erbium salt (erbium(???) acetate), stannous chloride, and aluminum potassium sulfate. The results indicated that the dyeing of silk in the presence of [Er(AO)(H 2 O) 7 ](OAc) 3 as the mordant provides the highest color difference, whereas the treated silk with [Er(AMH)(H 2 O) 7 ](OAc) 2 shows the most color strength value ( K/S ). Furthermore, all of the mordanted and unmordanted samples exhibited proper color fastness characteristics. Also, to study the fluorescence property of the mordanted yarns with erbium(???) complexes, the fluorescence spectra of these samples were recorded. The mordanted yarns with [Er(AO)(H 2 O) 7 ](OAc) 3 complex, showed green fluorescence emission at 540 nm. The [Er(AMH)(H 2 O) 7 ](OAc) 2 treated sample, emitted blue light at 420 nm. In the second part of this research, the effect of some simple lanthanide and the d -block salts (Yb(OAc) 3 ·4H 2 O, Dy(NO 3 ) nH 2 O, Co(NO 3 ) 2 ·6H 2 O, Cu(NO 3 ) 2 ·3H 2 O, Ni(NO 3 ) 2 ·6H 2 O, and Fe(NO 3 ) 3 ·6H 2 O), as well as their coordination complexes with acridine orange and azomethine-H on the colorimetric properties of dyed silk with natural dyestuffs from five plants including Madder, Juglans regia L, Pomegranate, Reseda, and Safflower, were investigated. The results showed that the use of natural dyes in combination with the transition metal complexes is a good strategy to reach a wide range of colors in the natural dyeing, which makes the natural colorants more attractive and opens their place in the market.