Open the plain flavor agar-agar envelope, divide into 3 equal measures of agar-agar powder. Pour 1 1/4 cup milk to a pan, add in the first agar-agar powder, blue food coloring, and 2 tbsps sugar (or 2 sachets of sweetener), stir well. Bring to a boil, pour mixture into mold. Refrigerate until firm and set.
Repeat method twice more for the rest of the plain agar-agar, only this time use another color. Refrigerate until totally set. Make slices or cut cubes the firm agar-agar, place in another mold.
Prepare the chocolate agar-agar as directed on the package (this time use water not milk), then pour over the mold where the colorful agar-agar are. Refrigerate until firm.
Serve chilled with vanilla sauce.