Thoroughly wash a 6-inch length of 1-inch diameter PVC piping with soap and hot water. Allow to air-dry completely. Do not use a rag or paper towel to dry, as this will leave behind lint and residue that may clog the marker.
Using a small amount of plaster or other molding compound, create a thin wall halfway through the length of the PVC piping. This will separate the two colors of paint so that each one comes out of a different end.
Allow the molding to dry overnight. Alternatively, a piece of cut plastic or rubber may be glued in place to separate the two ends of the marker. Be sure that the wall is free of holes and leaks.
Holding the PVC piping over a drain or sink, pour a small amount of paint thinner into one end of the marker. The paint thinner is what will allow the paint to run through the tip of the marker. For the best results, experiment with the amount of thinner used to find a good balance between too drippy and too thick.
Fill the end of the paint marker with paint until it is almost full. Leave 1/2 inch to 1 inch of space between the paint and the top of the marker.
Cut a strip of felt or another thick material roughly 1 inch wide and 10 inches to 12 inches long. Roll the fabric tightly into a cylindrical shape. If rolled too loose, paint will run out of the marker when it is not in use.
Insert the rolled fabric into the end of the marker. If necessary, the fabric may be glued in place. Place a PVC end cap on the marker, and turn it upside down.
Pour a mixture of paint thinner and paint similar to that of the finished end of the marker into the empty side using a different color of paint.
Cut a second strip of fabric 1 inch wide by 10 inches to 12 inches long, and roll it tightly. Push it into the end of the marker or glue it in place.
Cap the second side of the marker with a 1 inch diameter PVC end cap. Holding the marker over a drain or sink, shake well for two to three minutes to thoroughly mix the paint and thinner.
Remove one of the end caps, and use the newly functioning marker to mark the cap. This will help you distinguish between the two sides. Remove the other end cap and mark it with the other color.