With the use of petroleum-based synthetic dyestuffs, the search for alternative dyestuff resources for the coloring of textile surfaces has been increasing rapidly for the last 20 years, due to the harmful effects both on the environment and on the human body. It is possible to come across many studies of scientists in the literature on this subject. With this research study, it has been tried to create an alternative dyestuff source that can be used in coloring textile materials. In this study, it has been tried to create an alternative dyestuff source that can be used in coloring textile materials. In the study, a natural dyestuff mixture was prepared by combining purple and red onion skins and then pre-treated cotton yarn was dyed. Five different metal salts were used to make the dyeing process more efficient (KAl(SO₄)₂.12H₂O, K₂Cr₂O_7, SnCl₂.2H₂O, FeSO₄.7H₂O, CuSO₄.5H₂O). The experiment was designed so that the metal salt concentration used in the simultaneously-mordanting method was 1:1 with the weight of the material to be dyed. The dyeing process was carried out separately in the laboratory environment at 100°C for one hour. It has been observed that a wide color ranges from orange to light brown can be obtained as a result of dyeing processes performed by combining purple and red onion skins. After dyeing, the dyed samples were washed and unfixed dyestuff wastes were removed. Washing, rubbing and light color fastness tests were carried out after the cotton yarns dyed by using the simultaneously-mordanting method were dyed. The CIE L * a * b * and K / S color values of the dyed samples were measured with a Konica Minolta 3600 brand spectrophotometer. As a result, satisfactory results were obtained in terms of both color fastness values and CIE L * a * b * color values.