Colorimetric analysis is a fundamental technique for quantifying the concentration of a substance in solution. It is frequently used in primary and secondary education to enhance students' interest in chemistry, biology, life science, and environmental problems. The structure of the colorimeter is quite simple, i.e., a light source, a sample vessel, and a detector are arranged in a straight line. Therefore, a variety of handmade colorimeters have been reported. However, easy-to-make colorimeters lack portability and reproducibility of measurement, whereas high-precision colorimeters require soldering, which is difficult for beginners. To reduce these difficulties, this study proposes a new open-source colorimeter that can be fabricated easily and cheaply without any soldering. Electronic circuits were made by wiring plug-in electronic modules with connectors. The enclosure was designed to be assembled by simply inserting a laser-cut claws into the corresponding claw holes. The colorimeter was used to measure potassium permanganate solutions of different concentrations and its accuracy was verified. The results showed that the absorbance was measurable up to 1.8 for practical use and 1 for reliable use with the resolution of 0.01.