Peel the sweet potatoes thickly and cut into 1-inch (2.5 cm) rounds. Soak in water with baking soda for about 30 minutes to remove bitterness. In a small bowl, dissolve the matcha powder in hot water and set aside.
Place the sweet potatoes in a pot with enough water to cover, add vinegar, and bring to a boil over medium heat. When slightly softened, drain, replace with fresh water, and continue cooking until tender. Drain well and mash while still hot.
Transfer the mashed sweet potatoes to a pot, add the sugar, and cook over low heat, stirring with a wooden spatula until the mixture is thick enough to hold its shape. Remove half of the mixture and let cool. To the remaining half, stir in the dissolved matcha quickly over low heat to preserve its flavor.
Once cooled, divide each color mixture into six portions, making six green and six yellow pieces. Place one green and one yellow portion side by side on a damp cloth, then tightly wring the cloth and gently shape the mixture into a smooth form.