Begin by mixing together in a spoon over the sink one drop of yellow food color, three of blue, and seven of red. Because the strength of each color varies, you may need to experiment to get a true black.
Next, cut a paper towel or another kind of absorbent paper into 1″ x 1′ strips. Then use a toothpick to place a drop of black liquid about 3″ from one end of each strip. Hang strips in a jar containing a little water as shown. Ends of paper must touch water, but black spots must not. Bend upper part of paper strips over jar’s edge, and tape strips in position.
In a few minutes, black spots will move up the strips with the water and become a rainbow of color. Both the primary colors—yellow, blue, and red—and the secondary colors—orange, purple, and green—can be seen. Eventually, secondary colors disappear, leaving only primary colors. Then primary colors become separated on strips.
After the colors separate, it is possible to get them to form black again on strips. Begin with shorter strips this time, and don’t bend them. Instead, poke a straightened paper clip through upper end of strips—with black spots above the water—and suspend strips in center of jar with paper clip wire reaching across jar’s mouth. After the first color reaches top of strip, it can go no farther; soon the other two colors will join it to make black again—if you wait long enough.