Fold a sheet of paper towel in half and then in half again. Squeeze out a small dollop of each color of paint and then mix together to create additional colors for your tumblers. You can create different tints of colors by adding white. Or experiment with mixing the primary colors to create greens, oranges, and purples. Place a length of parchment paper on a sheet pan, and then place the glasses upside-down on the pan.