The textile industry tries to provide different opportunities to its customers. Because of this, novel technologies in textile finishing and the use of different fibers have great importance for the textile industry. In this respect, the use of casein fiber is of interest to both manufacturers and consumers. In this study, casein-based fabrics were pretreated by means of ozone gas. The fibers themselves are clean but have low whiteness degrees. Therefore, the fabrics were bleached by means of ozone gas but limited increases in whiteness degrees were obtained. After the bleaching process, a natural dye source, namely onion skins, was used for the coloration of the fabrics. The dried and milled onion skin was directly added to the dyeing bath as a kind of natural dyestuff without undergoing any extraction process beforehand. In this way, it was aimed to show the usability of a vegetable waste and, at the same time, to combine ecologic processing-dyeing with casein finishing. It was observed that with the use of onion skin, the coloration of the casein fiber can be managed easily.