Biocolorants and sustainable textile materials have gained growing interest. Traditionally natural dyes have been used for dyeing wool and silk, but demand for cotton and regenerated cellulose fibers increases because of their popularity especially in clothing. This study expands the natural dye research to one of the latest innovations among regenerated cellulosic fibers, i.e. Ioncell. It explains for the first time the dyeability of Ioncell-F fibers with biocolorants and compares the results with three other regenerated cellulose fibers, i.e. viscose, bamboo viscose and lyocell, using protein fiber wool as a reference The colorants from the food side stream, the yellow onion (Allium cepa cv. Settonia) and the forest mushroom Cortinarius semisanguineus were used as dyes. The composition of colorants in each dye sources was analyzed in detail. Methods of exhaust dyeing (80 °C, 1 h), with alum, FeSO4 and tannin as mordants, and the high temperature high pressure dyeing (130 °C, 1 h) were applied. The color of the dyed materials was studied as CIE L*, a*, b*, C*ab, hab and the K/S (λ420/480 nm) values. The colorfastness to washing and light was examined according to the ISO standards The results showed that for regenerated cellulose fibers the strongest color was obtained with polyphenols in acidic conditions, whereas with the anthraquinones the dyeing results remained light revealing hindering forces between the dye and the fiber. The colorfastness values were at highest for iron and aluminum mordanted samples, which indicate the metal ions′ ability to stabilize the organic compound′s structure and form strong coordination bonding between the fiber and the dye. The colorfastness varied from poor to moderate. The exhaustion levels between different regenerated cellulose fibers varied very little. The search for sustainable colorants, fibers and their applications in long lifetime artifacts is strongly supported by the sustainability goals, and colorant production in connection with forest and agricultural industries support this emerging field.