A multi-ion chromogenic sensor based on a terpyridine moiety was developed for the semiquantitative, visual, and sensitive speciation analysis of Fe2+ and Co2+ ions in water. Each metal ion exhibited a different color, and significant color evolution was observed by the naked eye, resulting in semiquantitative visual detection. A smartphone was used for visual detection by identifying the RGB values of the probe solutions. The application of smartphones shortened the detection time dramatically and reduced the detection cost. This method provides a new strategy for the semiquantitative detection of heavy metal ions in water samples.