Alum helps capture the color and yarn to make colours brighter. And tamarind is the acid-alkaline, with water color water color can cling better fabric fibers.
Alum helps hold the yarn color and allows bright light. And tamarind to adjust the acid - base color to the water. So that water can adhere the fabric better.
Alum help capture the color with yarn and help the color bright. Tamarind and will adjust the pH to water color. So water color can cling to cloth fiber well.