Red onion, "Allium cepa," juice can dye a cloth sample green upon the addition of alum, aluminum potassium sulfate, and this sample can turn red when subjected to an acidic solution. When the red-colored sample is washed with a neutral or alkaline aqueous solution, its color turns green again. These experiments illustrate color changes that can be used in activities to aid understanding of metal coordination and acid-base chemistry. The addition of alum (aluminum ions) to red onion juice leads to the formation of a coordination complex causing the green color and generating a pH-responsive dye. This paper introduces a series of experiments, such as the use of a simple and low-cost method to dye a cloth sample green using red onion skin and alum, color variation of the cloth sample between green and red by adding readily available acids or bases, and use of commercially available chemical reagents to separately dye samples yellow, blue, and green. By conducting these experiments, first-year undergraduate students in the engineering faculty could reinforce their knowledge they had learned in the classroom through practical experience.