Fill a tall cup or vase with cool water. Add drops of any color food coloring, such as blue, red or green, to the water until you reach your desired shade. Use approximately 25 drops of food coloring per 1/2 cup of water to get a rich, vibrant shade. Add less food coloring drops for a more muted shade or more coloring for a deeper hue. Swirl the cup or vase gently to incorporate the dye into the water.
Cut fresh, white mum stems at 45-degree angles, 1 to 2 inches up from the bottoms, to encourage maximum water absorption. Use a sharp knife, shears or floral clippers to make clean cuts.
Place the flower stems into the vase. Allow the blooms to stand in the dyed water overnight or until their petals are filled with color; the dye will reach the flower's petals as the water is absorbed through the stems. Keep the stems in the colored water for up to 24 hours to achieve varying degrees of color intensity.