Markers are a vital part of the artist's toolbox and have been used for everything from simple sketches to award-winning gallery pieces. Permanent markers are expensive, but some brands, such as Pilot, are refillable, so you don't have to buy a new pen every time the ink runs out. And you don't even have to use ink made by the manufacturer. You can make your own ink relatively cheaply.
Put on all protective gear before you begin. Choose which paint color you want to use for your ink, and fill your container with no more than twenty milliliters (mL). Add no more than ten mL of paint thinner. Stir the two together thoroughly, testing the mixture for thickness. Add thinner in five mL increments and stir thoroughly until you've achieved the thickness of ink that you desire. You are looking for a fairly watery mixture that will soak into cloth and marker tips.
Dip your brush into your new ink and test it out on paper. Look for thickness, consistency, color, and ease of application. To alter the color, simply add another type of paint, stir it in, and add paint thinner to maintain consistency. Be mindful of consistency: too much thinner will make the ink drippy and sloppy, while too little thinner will make the ink hard to use and may even ruin your marker.
Open your marker's reservoir and clean out any remaining ink and pigment with paint thinner. Using the funnel, carefully pour the ink into the marker. Remember that the reservoir can hold only 30 mL of ink. Once the reservoir is full, close it, cap the marker, and shake it vigorously to allow the ink to soak into the tip. Test the marker on a sheet of paper to make sure that the ink is fully soaking into the tip.
As you become more comfortable, try different kinds of indelible ink.
Always keep chemicals stored properly.
Some recipes advise using brake fluid. This is extremely dangerous and should not be done.
Do not use markers for graffiti, which is a crime and can have serious punishments.