Capillary action is the force that is applied to the molecules in the water as they are absorbed by the towel. The pressure gently pushes the water all the way through the towel, and down into the jar next to it.
This is how the colors remain in primary in secondary colors, rather than continuously traveling from jar to jar, which would make all the jars turn brown.
When the paper towels are rolled up and placed between two jars, they exemplify capillary action, which is how liquid can move up something, rather than follow the usual pull of gravity and pull down.