The grafting of specific functional groups has been drawing attention later as a simple and effective way to modify materials surfaces to induce a particular response without significant changes in bulk properties and even very small impact in some surface properties. In the case of anionic group grafting, colorimetry analyses using complexed cationic stains are a fast way to determine the grafting density. This technique is especially interesting for the quantification of grafted species such as sulfonates or carboxylate groups. Although simple, this method involves a certain number of crucial parameters and small deviations could lead to unreliable quantification and consequently lead to a performance below the expected. To evaluate how the parameter choice, or errors, could influence the final quantification, poly ε-caprolactone samples were grafted with poly(sodium 4-styrene sulfonate) and stained with toluidine blue O in different conditions of pH, incubation, and decomplexation time, and light exposure. All four studied parameters have shown a stronger influence on the final measured grafting density value.