Berry juice can stain, so be sure to place newspaper over your work area. Wear aprons.
Put 1/2 cup of blueberries in a bowl. Add 1/4 cup of water. Mash with the potato masher until juicy.
Put your strainer over the second bowl and pour in your mashed berries. This is your purple (blue) watercolor. Repeat this process for your red berries.