When it touches the black line, you'll start to see some different colors. Leave the paper in the water until the colors go all the way to the top edge. Use a clean, dry coffee filter circle. Use a marker to draw a black spot in the center. Put the circle on a saucer, and put a few drops of water on the spot.
Most nonpermanent markers use inks that are made of colored pigments and water.
On a coffee filter, the water in the ink carries the pigment onto the paper. When the ink dries, the pigment remains on the paper. When you dip the paper in water, the dried pigments dissolve. As the water travels up the paper, it carries the pigments along with it. How fast each pigment travels depends on the size of the pigment molecule and on how strongly the pigment is attracted to the paper.