The present work describes a simple hands-on experiment kit for colorimetric quantification of ferric (III) ion (Fe[superscript 3+]) in an aqueous medium using anthocyanin extracted from "Ruellia tuberosa" L. as a green indicator. The extraction of a high amount of anthocyanin was easily accomplished by using only hot water instead of an organic solvent. The formation of the colored Fe[superscript 3+]-anthocyanin complex occurred on a homemade 24-well plate and the generated color was captured by a smartphone. The increase in color intensity was measured in the red, green, blue (RGB) system by the ImageJ software under the optimum conditions. The developed method enabled quantification of Fe[superscript 3+] at low concentrations with the detection limit of 0.03 mg L[superscript -1] and provided the linear range (0.05-2.0 mg L[superscript -1]) with good linearity (R[superscript 2] = 0.9985) with Fe[superscript 3+] concentration. The concentrations of Fe[superscript 3+] in water samples determined by the developed method were not significantly different from those measured with UV-visible spectrophotometry at a 95% confidence level. In addition, the extracted anthocyanin stored at 4 °C was stable for two months. This hands-on experiment was implemented as a 2 h activity for 30 grade-12 students in which they were asked to determine the concentration of Fe[superscript 3+] in a water sample using the smartphone-assisted colorimetric method. The students' understanding of the related concepts of oxidation-reduction and determination of iron was collected by a diagnostic conceptual test. Having participated in the experiment, the students were found to have significantly improved understanding of both concepts.