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Prepare the Dye
Use between 30 and 45 drops of food coloring per 1/2 cup of water to make sure that the colors show up on the carboard tube.
Once your work space is clear and covered with newspaper, you should begin to prepare the dye that you will use to color the cardboard.
Start by adding 1/2 cup of water to each of the four containers. Then, in each one, place 30 - 45 drops of food coloring, so that the dye is very dark.
Since cardboard is brown rather than white, the dye must be very dark so that it will show up once the tube is dry.