A new image analysis technique is proposed to determine the macromixing time in a transparent stirred tank. It consists of capturing on video a decolorization process by using a fast acid-base indicator reaction and employing image analysis to quantify the color evolution. The color change is quantified by means of individual thresholds on the RGB color model and provides a direct measurement of the macromixing evolution as it can be seen by an operator in front of the vessel. It is shown that this technique removes the subjectivity of the estimation of macromixing time by the naked eye, has a high degree of reliability and repeatability, and can yield accurate macromixing information by considering the possible presence of segregated regions and dead zones. Moreover, applications show that the macromixing curves bring new insights to study and compare mixing efficiency of different impellers or multiple impeller mixing systems.