The valorization of onion (Allium cepa) solid wastes, a 450 000 tonnes/year waste in Europe, by a green extraction method is presented. Polyphenols of onion solid wastes were extracted using eco-friendly solvents, such as water and glycerol. The 2-hydroxypropyl--cyclodextrin was also used as a co-solvent for the augmentation of the extraction yield. The process has been optimized by implementing a central composite face centered design of experiments, with two replicates in the central point, taking into consideration the following independent variables: glycerol concentration, cyclodextrin concentration and temperature. The assessment of the extraction model was based on two responses: the total pigment yield and the antiradical capacity. LC-MS analysis was also employed in order to identify polyphenols and colorants of the obtained extracts. The main polyphenols found were quercetin and quercetin derivatives and the main colorant was cyanidin 3-O-glucoside. The extract was also tested as a food colorant in a yoghurt matrix. The onion leaf extract was found to be a stable natural colorant and could be utilized as an alternative ingredient to synthetic coloring agents.