Create your model of a volcano, working or not, out of clay, salt dough or paper mache. If you want to make a working volcano, be sure to leave an opening in the center to place a reservoir to hold your reacting ingredients.
Look at a picture of a real volcano to determine how you would like yours to be painted. This will help to make your volcano look more realistic.
Paint the volcano a blackish shade or dark grey using acrylic, tempera or other similar paints. Leave the area around the top of the opening of the volcano and where the lava runs unpainted.
Rinse your paintbrush, and then use white or red to paint around the opening of the volcano and a path where the lava flows. If you want to get more intricate, you can do white underneath and then a small amount of red from the inside of the volcano dripping out.
Rinse out your brushes once you are finished, and allow the volcano to dry over night.