Natural dyes have been a part of human life since time immemorial due to their non-toxic and biodegradability. Environmental and health hazards created by synthetic dyes once again proved the importance of natural dyes. In the present study, dye from Torenia sp was extracted by crushing the flowers with water and extraction was optimized with different temperature, pH and time. Silk yarn has been pretreated to remove dirt and improve texture. Biomordant (Tannin) from Peltophorum pterocarpum bark and Tamarindus indica seed coat were used to improve binding of dye to the silk yarn. Extraction of biomordant and mordanting process were standardized to study their effect on colour and fastness properties. The conditions optimal for dyeing such as pH, temperature, time and dye concentration were also standardized. Dyed yarn has been evaluated for its fastness properties such as light, wash and perspiration using standard ISO methods. The results showed good fastness properties with the application of biomordant. Phytochemical, TLC, UV-Visible, and FTIR analysis were done to investigate the pigment present in the dye. Results showed the presence of Anthocyanin group of compounds that are responsible for the blue colour of the flower and gives green colour to the dye upon change in pH. Since green coloured dye is not commonly available from natural source, Torenia sp flowers can be effectively used in textile industries as an alternative for synthetic green dye.