This analysis intends to unveil the effect of metallic mordants on colorimetric properties of ecologically dyed cotton fabric using banana floral stem sap (BFS).Natural dye retrieved from banana floral stem by roller squeezer machine. Pre-mordanting action was performed by potassium alum, iron (II) sulphate, copper (II) sulphate and tin (II) chloride. Pre-mordanted specimens were dyed at 100 ℃ for 60 minutes. Dye fiber binding actions was determined by FTIR-ATR spectra. Effect of metallic mordants were calculated using CIE L * a * b * color space in terms of colorimetric properties viz. color coordinates value i.e. lightness (L*), redness (a*), yellowness (b*), color saturation (C*) and hue angle (h 0); color strength (K/S) value; degree of color levelness and brightness index (BI). Though iron (II) sulphate treated samples exhibited height color saturation but uneven physiognomy was observed. Among them best colorimetric appearance yielded in case of alum. This reports forecast a sustainable pathway for effective utilization of agricultural waste i.e. banana floral stem sap for coloration of cotton fabric.